My CH only works if I have my HW on as well - help!?

I have a Potterton Netaheat boiler and Potterton Ep2000 controller. If I set the CH to a timer switching or to 24 hrs or even hit the override button nothing happens. I can hear the thermostat click when I hit any of these functions but nothing happens.

However, if I set the HW to 'once' or 'twice' or 24hrs and then put the CH to the same setting, the boiler fires up and I have heat.

It used to work if I pressed the 'over-ride' HW button and then the 'over-ride' CH button. However, I have noticed that the 'over-ride' function no longer works for either CH or HW.

The diverter valve appears to be moving as follows:
From turned off to HW on, the valve moves and HW's on
From Turned off to CH on, the valve moves but no CH
From turned off to CH/HW, the valve moves and we have both.

Can anyone help?!
Thanks,
Lesa