I can't say that I've ever heard of a "Damp" certificate, Mcvj. I can't imagine how even a registered Surveyor could ever discharge any property as being "damp-free", given the circumstances you outline.
You know, of course, that you buy a property "as seen". That includes defects.
It does sound a terrible bodge. Unless the terms of the contract have been broken, there's little or nothing that can be demanded of the sellers now.
You really do need good legal advice (CAB should help here.) I'd love to know who came up with the damp certificate. A damp-proofing company would usually use this as an opportunity to emphasise a damp problem to get the remedial business.
Whoever provided the certificate should be liable via their insurers. Can you be sure they even have insurance. So much of this boils down to simple "matter of opinion".
On the positive side, I'll bet that, once the roof is fixed, the "damp" problems will likely disappear with normal heating and ventilation. Even badly fitted windows etc can usually be put right fairly simply.
Good luck