This is the solo 20 deal on there site. You get 500 minutes and unlimited free texts to any network, all for £20 a month. Its not a long term contract either, can cancel with a months notice. Also saw that (could be wrong) you can have a days unlimited internet for £1, says in the plan. I think it's a great offer. Checked a few post codes to see if t-mobile had a good signal and they all said very good.
Unlimited free texts offer ends 30th june 2008 and can be applied for over the phone, internet or in store. Others availible more more minutes etc but this is a good simple deal, for a good price.