Hi Psybbo,
In my opinion and experience of damp ( I had some damp in one of my first apartments when I was teenager a few years ago ........OK, stop sniggering.....ALOT of years ago then For Funks Sake ) painting over it with a damp course protective paint before normal painting does'nt really work out the problem.
Why would the builder have to dig anything up outside, Psy?......the problem may be ventilation related and I would bring back the builder and ask him to have another look at it for you. How long has it been there? Do you regularly open the windows for ventilation and air?.......If he knows his trade he will be able to find the reoccurrence of the dampness......I hope our very own hunk The Builder or any other good Building folk see this and may be able to advise you more than me as I deal and specialize with playing cool dance tunes and really entertaining people's bodies and feet, so I hope you sort out the problem and very soon before the really cold weather sets in, psybbo.
Best of Luck to you :0)