potterton performer 24kw HEcombi?

Hello ,is there any quallified heating engineers out there who may know wat this fault is.
no hot water,the push rod on the dhw pressure differential assembly is working when i call for a hot water demand.which in turn is switching the pump on.
The 3 way valve assembly push rod is then supposed to rise to turn on the fan on etc.
this is were i have become stuck.
lucky for me having plenty of spare parts i have replaced the inlet valve assembly,ch 3 way diverter inc diafram,,dhw pressure differential assembly+ diafram,plate heat exchanger,pump,blown through all sensor pipes,checked secondary heat exchanger for blockage ,checked main heat exchanger for blockage,checked for 230v at pump.ho and nearly 15 hours labour.
the boiler is only 2 years old and x100 inib was added.
the push rod in question does move in ch mode ok.as soon as any demand for dhw is made the rod moves away from the switch.
if i activate the dhw switch with my finger everything works fine until you open a tap.
somebody please please help
kind regards
steve