Old, old problem, Tiggs .......... surveyors with "damp meters".
I've seen meters indicate damp after just boiling a kettle etc etc.
They constantly give false positives, and are generally the last resort of those without sufficient practical knowledge. "Proper" surveyors should know better.
On the face of it, I would be reassured by what your roofers have found... ie nothing.
Hold your nerve Tigger. I've come across this a hundred times.
Challenge the report, backed with your roofer's opinion. You can do no more. Even if there were a bit of damp, in this situation there's no way it should be a deal-breaker.
I guess this is upstairs in a bedroom. Their is "damp" everywhere, even in a modern house. It comes from the fact that people live in it.
The meter most likely showed the result of a little moisture in the air settling and condensing on a colder surface.
I'll further bet that the flue hasn't had a fire up it for some time ;o)
Chuck it back at the buyers Tiggs. It's their decision. The "process" is just a matter of everyone involved being seen to do their jobs and tick their boxes.