ideal FF270 ticking but no heating. Pilot light OK?

My Ideal FF270 is installed to supply CH only and sometimes works perfectly. However, in really cold weather, after the initial start up and system getting up to boiler max temperature., the boiler stat switches the boiler off. The pump continues to run as the room stat is still calling for more heat. When the boiler stat resets the boiler starts ticking continuously but will not fire the main burners. The pilot light can be seen to be alight.

If the boiler is switched off and immediately switched (toggled) back on, full ignition is acheived and the boiler start working properly again, though when the water temperature causes the boiler stat to switch in again the cycle repeats.

The fan is running with out any strange noises.

Any ideas??

Anthony