Everything works fine in Chrome.
However in Internet Explorer and Firefox, submitting the check out form results in an error message from Realex/Global Iris:
505 You cannot use this service from there.
Global Iris say that their logs suggest that the submission URL is somehow being masked - hence it doesn't match the referral URL that they have on record, so the request fails.
edit: if I type document.referrer into the browser console, Chrome shows http://www.domain.com/cart/checkout, while both IE and FF show nothing.