Potterton 50E Neataheat profile - No Central Heating? ?

Potterton 50E Neataheat profile ( conventional heating system)...

Hot water is working on demand.. ok
Central heating not working on demand…?

Could it be the overun thermostat on the boiler ?
or
Gravity DHW/pump overrun thermostat on the boiler?
Parts changed over the 3 years..

1).New PCB electronic control ( Located at the bottom of the boiler) - 1yrs

2).New Gas control valve - 3yrs

3). New motorized valve & thermostat - Water cylinder tank -1 yrs

Also Over flow - tank in loft has hot water venting into it?
All Rads bleed- no air in system?

I can hear the pump running and all pipes around water cylinder are hot very hot?

HELP PLease

Anthony
January 2011