Heating comes on when hot water tap is running?

The problem with my Halstead Ace boiler is that when i run any hot water tap the heating circuit is also getting hot.
The plumber has swoped the diverter valve but the problem hasn't been solved. The plumber noticed that when the pin pushes onto the microswitch in hot water mode the pin on the diverter valve is also pushing up onto the Ch microswitch, is this normal or should the CH pin stay still? Could this be allowing both circuits to get hot and if so what is the cure?Anybody help!. Thanks