myson orion fanfare 40 si?

myson orion fanfare 40 si has a elctronic igition. when the hot water or central heating or both are set on timer. sometimes the boiler do not kick in. I can hear the fan going. other times the boiler does come on but after rest the does not fire up and I can hear the fan running.
This also happens when I do it manually too.

some times I turn it off and on again from the clock the boiler starts. if that don't work I would turn the room themodial to zero then turn it up till it clicks which can kick fire the boiler.

Do you think it's the thermocouple?

You kind advise will be much apperciated.

John King.
October 2011