After seeing some good deals with Prepaymania I came across another which I think is Hot.
Nokia 1208 on T mobile pay as you go including £20 top up for £19.99 after using promotion code. Plus its free delivery too. Normal selling price without code is £24.99
Use promotion code MG9183 on the basket page and your basket price will go down to £19.99 so you are getting a Free phone basically as your only paying for the top up.
I have seen similar offers of this phone on different networks but not any decent one on T-mobile. (This offer is extended until 31st of May 2009)