At this stage, I would be tempted to pack your bags and head off. I am not sure , I would even bother telling them. On reflection, send them the keys and a letter setting out your attempts to resolve the matter.
We have had a set date for leaving from the beginning so it is now looking like abandonment is going to be the remedy. We set a date for reply with a reasonable proposal but there is a good chance the buyer is going to say forget it if there isn't any reply within a week. The bank knows all this and although I have offered to meet in person to help the process along, they seem to be intent on dragging their feet.
I'm pretty much prepared for the nuclear option and am documenting accordingly.
Thanks for everyone's feedback.
PS I paid everyone involved (except the bank) costs so I'm not screwing anyone over (again, except the bank) even though we don't really have the money.