Hot water but no heating?

Hi i have a fully pumped two valve system which until recently has been working reasonably well, I say reasonably because the boiler cuts out occasionally via the overheat cut-out. However the main problem is that when the programmer is on with both circuits illuminated we only get the hot water side working with the heating valve failing to open, the only way to get the rads heated is to open the valve manually, but as soon as the cylinder is up to temp the boiler goes off even though the roomstat is calling for heat and then wait for the water in the cylinder to cool befoere the boiler comes on again. Not sure if a programmer problem or valve.
Glow-worm Ultimate boiler
Two Honeywell valves
Grundfoss pump
Potterton EP 3002 programmer
Honeywell roomstat