Unlocking phone at www.unlocks.co.uk?

I have a Nokia 6300 which is locked to Vodafone, but I have just started a SIM-only deal with O2. I have tried the new SIM but it doesn't work because my phone is locked. I know there are lots of websites out there that offer unlock solutions but I am wary of handing over my money unless I know the site is reliable. Has anyone used www.unlocks.co.uk before, and if so, did their codes work? Or have you got a better suggestion? Thanks everyone for any help you can give!