This is from the Consumer Direct website

Your legal rights:
Ticket sellers are legally required to:

1. Provide price information in advertising that does not, or is not likely to, mislead
2. Provide clear, honest and unambiguous price information at the point of purchase
3. Confirm seat location
4. Despatch tickets in time to attend an event
5. Refund all monies paid or provide a substitute for indoor events cancelled by an events promoter or producer where tickets have been bought by phone or via the internet
6. Give you information on the face value. This can be provided orally or in writing.

Remember, if you have bought tickets over the internet, cancellation rights that you have when purchasing other goods are not available. However, you are still entitled to a refund if the supplier fails to perform the contract.

So it seems that Ticketmaster are obligated to tell us our seat numbers, we need to all send letters to them via their website. I did last night, but no answer as yet.

it doesn't mention in those legal rights the timescales involved in advising that information, or how specific it has to be

thus you have been advised a rough idea of there the tickets are going to be, that could sufficient to comply with the law itself (ie. within floor or mid/lower tiers)

however, ticketmaster have said they will allocate all seats before the show. as the law doesn't specify when they have to do this, it's probably going to be acceptable to advise that information at any time before the show. TM will advise the seating location at least a few days before the show when they despatch tickets, so you will know in advance where you will be sitting

you don't need to send letters to them, you just won't get a timely response, if any at all. you will be allocated seating before the show and thus number 3 will be covered