baxi 105e not changing over to central heating?

boiler appears to be stuck on hot water even though central heating his selected and time clock and thermostate are made repair man has changed both diagphrams on diveter valve as he thought that this was the problem as soon as the top dhw micro switch is removed brieflythe boiler changes over to central heating the central heating led comes on and the boiler fires up and runs ok untill an hot water tap is turned on sometimes it changes back but most of the time it stays on hot water
their are no fault s showing on leds the plunger on the micro switch is free and not sticking the diverter valve operating head appears to work ok sometimes if you switch the main supply off and on then the system works ok for awhile again until an hot tap is turned on then the fault returns