Thanks greensgr. Unfortunately I did spend a very long time trying to get in contact with BA - first email (which they ignored), then a letter to customer services (also ignored) and telephone calls, which had to be followed up with further letters after they didn't even refund the 50% owed. So it was all quite a big undertaking. I finally spoke to one very helpful person in reservations, but she said, quite apologetically, that neither she nor her manager had authority to do anything.

So my only hope is if I can show they are in breach of their terms and conditions, which would be breach of contract and so basis to sue them.