I've heard Cake is pretty fishy, but it's the only site I've played on so I don't really know. It's a pretty small site but they don't allow HUDs (will lock your account if they detect one) and they change their HHs every now and then to make it difficult to track your hands in say in PT or HEM. This supposedly keeps a lot of sharks and multi-tabling TAGs away.

Oh, you're from the UK. I've heard a lot of American players on another forum talk about how much they wish Fortune was open to US players as they say Fortune is pretty much the fishiest site. I don't know this personally, though. YMMV.