Ariston Boiler Fan Going On and Off...?

Just trying to get an idea of what might be wrong with our boiler...

The boiler is an Ariston Micro System. What’s happening is that the fan starts for a moment, the switch clicks, then the fan stops, then the fan starts again, the switch clicks again, the fan stops again etc. etc. This obviously means the boiler won't fire up so we're currently without heating.

The fact that the switch can be heard to click which in turn results in a change of fan behaviour suggests to me that it's working correctly. Removing the plug from the test outlet on the top of the boiler (that Ts from the fan housing to the switch) makes the fan keep running, but the boiler still doesn't light. I've measured the pressure on that test outlet and it seems to be as per Ariston specs at 0.5 mb (negative, i.e. fan sucking).

I'm thinking it can only be the pcb, but would value any thoughts from those more knowledgable so I can get an idea of repair cost. I've been trying to get an engineer out to look at it since yesterday, but none of the engineers I've contacted has yet got back to me.

Thanks in advance.