spraying the roof timbers for worm is a very common and easy job, not expensive.
If you are handy, pull out all the rubbish and old insulation (if it is thin and yellow) first, so the sprayers can do a better job. Even better, hoover put the loft as well.
With luck, the worm will just be in the warmest areas, typically around the loft hatch where warm air leaks up, and maybe over the bathroom (they like humidity).
Test all the timbers by poking with a flat screwdriver (not anything round and pointy as the marks will look like wormholes) in case there are any that have been significantly weakened and need to be replaced.
Some parts of Surrey have a pernicious insect called the House Longhorn Beetle and local sprayers will know the procedure.
Get a written certificate of treatment and guarantee. It will be worthless but will comfort mortgage companies.
As for the damp, do not approach anyone who sells chemical injections, as they will advise you to buy chemical injections. Most likely it will be caused by condensation or by simple building defects that a builder can fix, such as paving or soil against the walls bridging the dampcourse; or overspilling gutters, or a water leak. Condensation can be cured by opening windows, bathroom extractor fans, and by not draping wet washing around the house.