Damp wall behind plasterboard

Bought a cottage 9 months ago and working away trying to get the rooms redecorated. The smallest room had a nice surprise when we took up the laminate flooring.

We had noticed it sagging a little after we had moved in (surveyor missed it, covered with a bed). Took up the laminate to find a damp sheet of MDF and a completely rotten joist on the outside wall. Que lifting the rest of the floor and a new concrete floor put in in stages

Before I put in the last section near the outside wall, I noticed it still quite damp behind the plasterboard. I know the plasterboard has a foil backing, but other than that, Ive no idea what state the wall is behind.

Anyone have any idea what I should be looking at here to fix the problem. Will the foil backing keep the damp out or should it be ripped out and completely redone