flow diaphragm on end of divertor assembly, only changed a few divertor assemblies on this model myself as usually id rather have my nuts set alight, far less frustrating and less time consuming. Id check from underneath if poss to get ya head in there is a black microswitch box and a plunger that comes out from brass assembly. boiler turned off head under get someone to open tap, if plunger dont come out to hit microswitch, id phone a fixed price repair (from 1 of a couple big companies) in 'case' it is full valve fault as could be:
price of part + 2 hrs, maybe more + if they dont do it properly with all o-rings, chance of leaks.
If it hits switch then thats the most painful ruled out.
I hate cdi's combustion area ok but the pipework area is annoying. best o luck
a, July 2009