Have CH+HW but HW alone is intermitant?

I have Potterton Ep2000 and Baxi Bermuda boiler with two valves a pump and a HW tank .

I have CH but no HW or it runs intermittently I fitted a new thermostat to cylinder but this didn't work , when the HW is turned on I hear the valve move the pump doesn't start or boiler . With CH and HW on all works fine does anyone have any ideas please? As this intermittent fault is annoying. Thanks.

Martin