Thank you everyone for the replies, including the useful tips and tricks!
It looks like this is going to be a real pain to sort out. An AF agent (via Facebook) is now telling me "your ticket is non-refundable and non-changeable." But the "My Bookings" part of airfrance.co.uk tells a very different story:
In principle this problem should be fixable - I've got screenshots and emails that are unambiguous. But I fear the reality might be quite different. Where would be a good place to start?