1) An "active" infestation of woodworm in the roof and floorboards
I'd put money on it last being active a few years after the house was
built - flight holes are left after the woodworm have gone. I wouldn't
do anything at this stage - if you buy it, put some masking tape over
the affected area and check back in the spring and see if any new holes
have appeared

2) Damp behind a sealed up chimney
easy to fix with ventilation as others have said - air brick in chimney
is a DIY job and your local chimneysweep will be able to cap the
chimney for a few quid

3) Damp in a window frame
cut it out, treat it, paint it, job done

Most importantly remember a free "survey" by a "Specialist" (read
"Salesman") is worth exactly what you paid for it

Charlie