I know it was rubbish to begin with but it serves a purpose for light
work. The motor is running all the time but the pressure just drops a
bit but the water is still "jetting". The trigger appears to come fully
out . I wondered if there was a pressure valve in the unit which cuts
the compresser when the trigger is released?

My cheapie B&Q model was clearly pressure switched on the motor. The motor
would stop when the pressure got high enough. When initially turned on, the
motor runs for a short period to get the pressure up and then stops.

The trigger simply operates a valve on the head that stops the water coming
out. The motor soon turns off as the pressure rises. There is no "signal"
connection of any sort between the head and the compressor. If you have the
same model, I suspect the valve in the head has broken and is not turning
off the water.

Christian.