fred wrote:

This boiler gets mentions here quite a lot

I've come to know the 80SP quite well over the last few months...

The flow switch would be a prime candidate and easy to check, disconnect
power, turn on tap and check for operation of the switch using a multimeter
on ohms/continuity. Next best is probably the diverter valve,

In this design secondary water flow is sensed hydraulically by pressure
difference across a diaphragm (the "domestic diaphragm") which moves a
push-pin which operates a pivoted arm (the fulcrum arm) which in turn
operates the diverter valve mechanically. When the arm has reached full
travel a microswitch ("domestic microswitch") which initiates the firing
sequence.

Firstly check that the arm is moving full-travel when secondary water is
flowing - full-travel being far enough to operate the microswitch. If
not, I'll lay a penny to a pound that the problem is a knackered
domestic diaphragm (Vokera part no. 6882, GC no. 301-180). Replacement
is an easy job.

If the arm moves OK and the boiler isn't firing I'd look at the domestic
microswitch next - this is easily isolated and can be safely tested with
a multimeter with the boiler powered down, since operation of the arm
does not depend on a supply of electricity.

--
Andy