Gazundered at the last minute — any recourse?

Buyer just dropped their offer by £15k, three days before exchange. We've been going back and forth for 3 months. They claim their surveyor found issues (which our survey didn't flag) and that the market has softened.

We're in a chain and our onward purchase depends on this sale. If we refuse we're back to square one. Feeling absolutely sick about it.

Is there anything we can do or do we just have to accept it?