diverter valve not operating?

Hi
I am having a problem with my potterton performa 28i boiler.
Central heating works fine. Dhw giving cold water only.
When you open a hot tap, Dhw diaphragm opens and micro switch is made, boiler fires up and the temp sensor lights on the front panel go up from 30 deg up to 70 deg. but the diverter valve does not change position. No change in voltage on PCB [to move diverter]
I have tried different positions of taps to give different flow rates but still the same.good flow of water from taps
I have gone through the fault guide and tested and or changed the following items
Diverter valve works if you press down plunger manually and diverts water.
Dhw temp sensor 11 ohms cold 3 ohms very hot
Diverter head new
PCB tried 2 off brand new boards, not repaired.
Checked both microswitchs for continuity both ok
Both the pressure valve and the Dhw pressure differential valve are working fine and both microswitchs work ok.
I have removed microswitchs and manually operated them and still the same fault