Found it. I cancelled todays job because of the snow and decided to start hunting for leaks.
It was the supply to the dishwasher which was leaking and the water had tracked its way down a joint between two pieces of underlay. It is very odd how the wet patch is so far away from the leak but the carpet feels dry in the gap between the leak and damp patch.
My best guess is that I damaged the joint when I had to remove and refit the dishwasher before Xmas after the bloody cat brought a budgie into the house and released it under the washer.
Adam