Cool, I've got some good options there then, thanks guys. And you're probably right Raw, the reviews on Amazon were more than likely coincidental - I did think it was a bit odd. So I won't dismiss the 500W Corsair after all either.
Pato Rodrigues, as you may have seen I have a very outdated gaming PC, that's currently pretty useless and has been sat gathering dust while I've used a laptop instead. I'm going to have some money from several people for Christmas - with this I want to get it in semi-decent shape as cheaply as possible, with room for more improvement but still at a decent standard.
Over next year I'll be gradually improving it further with an SSD, better cooling, perhaps a case replacement and Bluray drive, that kind of thing. But for the time being I want it to be able to play something like Battlefield 3 well for example - something it currently runs at about 10 FPS on bottom settings I believe.
So from my research and all the helpful advice from people in this thread, the plan is to go for an i5 3470/3570k, fairly cheap mobo like a Gigabyte Z77 D3H, 8GB 1600mhz RAM from Corsair, Gskill etc., and a ~500W PSU from Corsair or one of the others suggested like Seasonic. I'll be recycling the case, HDs, DVD drive, sound card etc. If I can't quite afford a replacement yet, I'll have to stick with the ATi 4850 GPU as well, but I'd like to be able to replace this with an upper mid range kinda card, like a GTX 660 or ATi 7850/7870.
So the ballpark figure including GPU would be about £500ish.