Baxi 105e ch problem

[geordiebuilder]

Hello Gas folk,
I have a problem with the above mentioned boiler in that my DHW is functioning normally but my CH is not.
None of the usual fault neons are flashing, pressure is normal and I've checked that power is getting through the selector switch to the PCB.
That is the limit of my knowledge, I'm a builder you see. The boiler is 6 years old and non condensing.
If anyone can shed any light on this problem I would greatly appreciate it.
Many thanks.

[htg engineer]

abit more information would help, does the heating work sometimes or not at all ?

does the central heating demand light come on ? is the room thermostat turned up ?