I was going to say something similar. Stateside, it's SOP to get the property inspected prior to finalizing a home purchase. It's meant to catch things like this to protect the new owners(/would-be owners).
And of course IANAL, but from a few quick searches it looks like there should have been an inspection (survey) as part of the purchase process (England and Wales is the jurisdiction). If a bad roof was missed by the buyer's lender/surveyor/solicitor, caveat emptor. Sure, knowing that the roof needed repair might have changed the buyers' offer, but that's why inspections/surveys are conducted.
AND at some point the new owners have to accept that owning a home means they are responsible for repairs and routine maintenance.
On second thought, let's not go to TechReport. Tis a silly place.