Take the cupboard out and strip the wallboard out until the damp & rot is removed and leave to dry for a few days....

Using a piece of wallboard that had been used to line the bottom ofthe cupboard I managed to match up, cut and fit a replacement board.

A tube of Filler, No Nails and a Night later I refitted the cupboard and the result was as follows!.....

So easy peasy and now that pesky damp has gone..... or has it!