Thanks for all the feedback folks. I'm erring towards worldpay because:
1) I don't have to host the https server and worry about the security aspect. I can also customise the pages through a web interface. and 2) I don't have to have a Merchant ID which frankly just seems a pain. Datacash do look cheaper, but that doesn't include the Bank percentage, the https server + sysadmin, or the development (how ever easy) at my end. I'll try worldpay and see what happens, if we get even half what the MD thinks we will (some how I doubt it) I'll do some more shopping around at a later stage as the annual fee is just 150 quid (which includes 3 currencies) so we wouldn't be too tied in. Ask me after September and I'll give you feedback how easy / hard it was to actually impliment. Cheers Leo