(Oh yes, and did I mention a new entrant to the market from Scandinavia who offers a cheaper rate - but then says in its T&Cs that you cannot offer to make it cheaper for people paying cash, cheques or bank transfers!)
It's part of VISA and MASTERCARD international's conditions that you can't charge more for using a credit card, though they do allow discounts for paying in cash.
However they don't seem to take any interest whatsoever in enforcing that rule.