I got a good deal on mine from Payment Sense -
Card Acceptance, Credit Card Payment Processing - Merchant Accounts, Lowest Prices in UK - I get processing of all the major card types including Amex. I pay 2.2% on visa, master card and 33p on debit cards. I pay 50p if I have to make a refund and charge backs are £15. The terminal and set-up are free too. They aren't the cheapest, but they seem to be quicker than the other providers and they're tolerant of our checkered credit history. The card machine itself is £19.99 per month but that's on a 5 year contract. Plus they're also sorting out my merchant account with the Bank of Scotland at the same time.
If you do decide to go with them please let me know via PM beforehand as they have a massive referral incentive and I'd love you forever and ever!
LOL