"It's always been a very big part of me, especially when I was a little girl," she confirms. "I had an incredibly full life with my imagination: I used to have all sorts of trolls and things, I had a wonderful world around my toys, and invented people. I don't mean I had imaginary friends, I just had this big imagination thing going on. I didn't need any imaginary friends, because I had so much other stuff going on. I remember one time, I nearly burnt one end of the house down, because my trolls were having this sort of outdoor party, and decided to light a bonfire on the windowsill."

She laughs, an infectious giggle in which the young child is still discernible. "I wasn't normally a pyromaniac, but I think that was the one time my parents thought it was starting to move into a slightly different..." she adds, leaving the inference unspoken.

"Obviously, I was very alarmed, so I didn't do that again!"

What she did instead was throw herself deeper into her fantasy life and her love of music, eventually combining them to develop one of the most singular careers in all of pop.

Restlessly creative, yet shunning the promotional scaffolding routinely used to hoist product into the marketplace, Kate Bush has managed to find a way of being both instantly recognisable yet oddly indefinable, a paradoxical combination of attributes that has enabled her to extend that career indefinitely, way beyond the confines of most of her peers.

When she first wafted her ethereal way into our lives, back in the late Seventies, punk was transforming the musical landscape of the UK. But while those former punks now very much look their age, both personally and musically, she and her work have retained a distinctive vitality and freshness. Being reclusive and refusing to perform live has afforded the 53-year old a cloak of invisibility that acts somewhat like an elixir of youth.

Few, if any, of her peers from that era would dare take a 12-year sabbatical between albums, as she did following 1993's The Red Shoes, let alone return to as universal a chorus of critical hosannahs as she did with 2005's Aerial.

Another six years passes, and suddenly we are treated to not one but two Kate Bush albums in a single year, her recent retrospective "re-imagining" exercise Director's Cut now followed by 50 Words for Snow, whose seven tracks stretch unhurriedly across 65 minutes, its languorous ambience casting a spell appropriate to the wintry themes.

"I'd had this idea for some while to do a wintry album," she explains to me, "and pretty soon after I started writing for it, I homed in to the idea of snow. It just seemed such a fascinating subject that it was very easy to think of so many ways of writing about it. It's such extraordinary stuff, isn't it? Even a single snowflake, when you look at it under a microscope, is such an incredibly beautiful thing. And apparently they are all different."

The title-track does indeed feature 50 words for snow, the majority of them made up by Bush herself, fantastical coinages like "vanillaswarm" and "icyskidski" intoned with gentle thespian relish by Stephen Fry, while Kate cajoles him in between verses, "come on Joe, you got 38 to go!". Did she, I wondered, coach him in his delivery, or rely on his judgment?

"I doubt if anyone coaches Stephen about anything," she admits. "He's way ahead of everyone all the time, isn't he? Really, it was just a case of focusing on the tone of it, him finding what his character was, because he is an absolutely brilliant actor. And in a way, the softer he said them, the more beautiful they became."

Which is entirely apt, as snow is silent, the stealthiest of weather: you just draw back the curtain, and suddenly there it is, blanketing everything.

"And also, it has the most extraordinary effect on acoustics generally. That's another thing I love about it, because it puts this great muffler around everything." Did it take long to come up with the 50 terms?

"Well, it was quite easy to begin with, because the first few of those words already exist. I did a first draft, but they tended to be phrases – one of the originals was 'robber's veil', for instance – and I thought I needed to make up actual words, rather than have them all be phrases. It was quite difficult at first, but then I got on a roll. They had obviously been forming at the back of my brain, and I was in the right place. But I was still coming up with words about 10 minutes before Stephen arrived, so it was quite a close call."

Elsewhere on the album, Elton John duets on "Snowed in at Wheeler Street", a tale of two immortal lovers' paths criss-crossing through the ages, from Roman times to the present day; Kate's son Bertie takes the title role of a snowflake yearning for human contact in "Snowflake"; a yeti is tracked across the Himalayas in "Wild Man"; and in "Misty", a woman and a snowman engage in a love affair which, I suggest, is ultimately doomed to end in death for one or the other: either the snowman melts from the warmth of human affection, or otherwise the woman freezes to death in the snowman's embrace.

"Yes, or gets pretty bad frostbite," she agrees. "Although my money's on the snowman."

These songs all deal with empathy for figures that only exist in a half-formed, mythical manner – it's as if the fall of snow offers cover for these beings, allows them to take life under its blanket.

"I like that, that's lovely, yeah," Kate agrees. "Someone else observed that a lot of the creatures are mythical, for want of another word, fantastical creatures – even a snowflake, if you think of it as a living thing."

The songs' abstruse fictional strategies of myth, fairy tale and time-travel also reflect her frustration with the restrictions of straight narrative. This is a songwriter, let's not forget, who had wanted to use Molly Bloom's soliloquy from the end of James Joyce's Ulysses as part of a song – an aim she was finally able to realise when re-recording "The Sensual World" for Director's Cut.

"I was originally refused permission, which was very disappointing," she explains, "but I just got on and wrote something that was me trying to sound very poetic, but of course was nothing like Joyce. Then when I came to Director's Cut, it seemed like it would be mad not to ask for permission again, because they could only say 'no', and I was really surprised that they said 'yes'. It meant that I could have the new lyric, and give it a new title, 'Flower of the Mountain': it would have been wrong in lots of ways to use the same title, because of course it wasn't the same lyric."

Director's Cut was effectively Kate's attempt to re-imagine songs written years before forthe albums The Sensual World and The Red Shoes as they might have been recorded by the artist who made Aerial.

Part II

It's simply the latest, and some would say oddest, manifestation of the dissatisfaction with the musical constraints of pop which led her to become an early adopter of the world-music sounds, from uilleann pipes to Bulgarian vocal polyphony, that helped give such unique flavour to her music. Even as a teenager, she chose to open her debut album not with the Big Hit Single, but with the evocative lowing tones of humpback whale-song. Does she, I wondered, get easily bored with plain old pop music?

"It depends what you mean, because some of the greatest songs ever written in contemporary music are pop songs," she says. "'Tracks of My Tears', for instance, is for me one of the most beautiful pop songs ever written and performed. In a popular medium, you're going to get loads of stuff that is trite, but there'll also be some really special moments. I am aware that I've been around a long time, and ever since The Dreaming, I've been trying to move away from what I felt was a more straightforward pop-song structure. And particularly with this album, I've pushed it further, in that the song structures have opened up and they're much longer. To me, it's an evolving process, it's just stepped into a different stage."

Her melodies, too, have grown more diffuse on recent albums. These days, Kate Bush is probably more likely to be appreciated by an ECM [Edition of Contemporary Music record label] aficionado than a pop fan.

"Oh! Well, I'll tell you what, I don't mind that. I take that as a compliment!" It seems like she's searching for the less obvious, hunting down stuff that's not in full view.

"A bit like stuff hidden under snow! I love the comparison with ECM, because one of my favourite musicians, Eberhard Weber, was on ECM, and one of my favourite albums ever is his Pendulum [absorbing solo double-bass instrumentals], which I think is extraordinary: he has such a distinctive voice, through his instrument. That's the kind of music I really like to listen to."

Many ECM records occupy the quiet grey area between chamber-jazz and contemporary classical, a space into which Bush's music seems to be moving. Had she, I wondered, ever considered classical composition?

"No, I haven't, really. Though I quite like the idea of emulating classical music in what I do. I'm not able to orchestrate, but I can certainly direct, and give ideas. Although this isn't a classical album at all, in some ways it holds elements of that."

One of the reasons for Bush's long absence from the public eye was her decision to prioritise the obligations of parenthood.

For earlier generations, the pram in the hall was viewed as the enemy of creative promise, and as the silence from her Oxfordshire home grew longer, it certainly appeared that way for her, from the outside at least. And the arrival of her beloved Bertie in 1998 did force her into devising new ways to work, so as to afford her as much time with her son as possible.

"I had to learn how to work in a completely different way, in short snatches of time, which was so not how I'd ever worked before," she says. "I couldn't do it at first, I had to learn how to do it. It was an extremely positive thing, because for the person, as well as the person's work, it's good to experience shaking everything up and doing something completely different. I love being a mother, I think it's the best thing I've ever done, and I personally feel that it's had a very positive effect on my work. I think it's an encouraging force for creativity, it feeds creativity – it did for me, certainly."

It's perhaps understandable that Kate Bush should take so enthusiastically to parenthood, given the enduring fascination with childhood in much of her work. Indeed, in the past she has admitted that even though an adult, she still felt like a little girl in some ways.

"I have a theory that there are still parts of our mental worlds that are still based around the age of between five and eight, and we just kind of pretend to be grown-up," she explains. "I think our essence is there in a much more powerful way when we're children, and if you're lucky enough to be treated reasonably well, and can hang onto who you are, you do have that at your core for the rest of your life. I guess that's what I meant, really: it's not that I actually think of myself as a little girl, but she is right in my core."

No, it's Grayson Perry that thinks of himself as a little girl...

"Well, I think he looks delightful, it's really sweet," she laughs. "But if he wasn't that talented, perhaps he wouldn't be so much in the public eye anyway. I like that there's this kind of eccentricity, which can be one of the more loveable sides of the English spirit. More power to him, I say! I think it's great for people to delight in their eccentricity." And indeed, it may be helpful for some artists to retain that childlike spirit to sustain their creativity, at the risk of appearing ridiculous.

"Yes, and also, if you've been lucky enough, as I was, to have a really lovely childhood, there's also a base of trust that you work from. Even though as an adult you have to be cautious – you know you can't go around trusting everything you hear, and so on – but if you have that element of trust it's very good for the creative spirit. Okay, so you're going to get beaten up a bit, but it's so much more preferable to living in a world of suspicion and cynicism, where you're trying to protect yourself all the time. You may get hurt, but you remain open, and then you can trust again."

Kate herself, of course, braved the risk of ridiculousness right from the start of her career, when her willowy, mime-derived performances, theatrical presentation and creative dancing, along with her stratospheric vocal register on "Wuthering Heights", proved a gift for impressionists, who would don fright-wigs, squeak shrilly and wave their hands around spookily in imitation.

"Understandably!" she chuckles, generously. "I think at that point it was all just funny. Everything was hilarious, really. Nobody expected the success – that first album wasn't expected to be successful at all, and that it should be so very successful was a huge shock, and just fun, in a way." Similarly, contrary to the impression most people have, she found performing live great fun.

"I enjoyed being a performer, I enjoyed the tour that I did, it was very exciting. And I enjoyed performing the videos, and hopefully, trying to find creative ways of presenting dance, and trying to make what I hoped looked like little films. But I experienced celebrity with the first hit of success: I could see that was what being famous was, and it wasn't what I'd been working towards. It was sort of fun, but it was taking me away from what I really wanted, which was to be a writer.

"So quite early on, I made the decision that I wanted to spend more time making records than doing promotion, because my desire wasn't to be famous, it was to do something interesting from a music point of view. I suppose I always thought I'd tour again some day, and it just didn't happen, I just went off along this path which led me much more into a studio environment where I became involved in production, and I really started to enjoy working on visual pieces, and gradually I came to prefer standing behind the camera, that's what I get the buzz from."

"The driving force, for me, is my desire to create interesting work. I don't really want to promote myself, I want to promote my work. The work is the interesting bit. I don't think that I'm that interesting."