I've just upgraded my phone contract to the Sony Ericsson K800i on O2, but in Mid-Feb I bought a w900i from eBay at about £350. (I get bored with phones pretty quick!)
It works really really well but I have knocked it about a bit. If you don't mind some scratches it's a really great phone and I'm flogging it. Ideally looking for about £150. It's unlocked to any network and it's the Black colour that's not actually available in the UK (I don't think). Vodafone do the ugly White one on Contract :-)
I have the headset it came with and can also throw in a Tesco Bluetooth headset FOC. I think I also have an O2 PAYG SIM card lying around somewhere that I can throw in.