i got a pretty good one on newegg for $20 if your on a budget, but others know better ones that cost more. that could deffinately be your problem (its the PSU that came with your case then right?) i'll be back with a link in a sec. newegg ships to the UK right?
EDIT: heres the one i use, it works just fine for me, but i use a VIA chipset, so i ddont know if this would hold up at higher levels of OCing because this hasn't held me back yet... know what i mean?