Well in the end I went to the local O2 store, and got a £30/month contract (400mins, 500txts) with the phone for free. 18month contract, but not really that fussed.
Phone came with no charger, so another trip back into nightmare shops (v.busy) and I had a complete phone - it's really nice, although I think I have a problem with the video playback - the demo's are terrible, but any video I take is ok....
I've dealt with o2 online before, and it was a bit of a mare, so I decided that the shop contract was enough for me - the online version simply gave me another 500 txts, but I won't use the 500 I get anyway, so wasn't really worth anything to me.
Thanks for all your help!