yep go to Asda they do a nokia for £19:97 then take it to the shop on the market and ask him to unlock it and put your card from the old one in the new phone.
it will cost you £5 to have it unlocked.
benipete a member on here has just done this and its a nice phone too
have a look at the link:
Asda-phones.co.uk: Nokia Pre-pay Mobile Phones