Worcester Bosch 240 RSF not firing?

Worcester Bosch 240RSF combi boiler.
Soft water, high mains pressure.
Here's the problem:
I had replaced the pump head as the old one's bearing were worn.
Simple enough job: bled radiators, vented pump. Heating & hot water from the kitchen taps operating nicely. The cold pressure was about 1 bar and when operating was around 2 bar with no fluctuations.
All OK for a couple of days.
Then when running a bath, the water was initially hot and then cut out. Tried with all the rads off, but no change. Now the heating doesn't operate either.
So, although pilot light is on, it doesn't fire up when demand is created.
I've tested the pump electrically but it's fine, so I think it's bad luck and a different fault.
I've also noticed that the green 'demand' light on the front panel does not light up regardless of rad positions or hot water demand, and the power light has a just discernible 'flicker'.
The problem I would guess is electrical, but how do I decide where the fault is? Would there be any point in changing the microswitch, or the diverter diaphragm?