during a job on my placement, fault on ariston eurocombi, i think it was
a a23 or 24 mffi, no hot water but heating ok.

diverter valve? well the pin that contacts the micro switch was not
moving fast enough, if at all, to engage switch and thus boiler not
firing and water wasn't getting hot.

now this is where i was thinking instead of jumping in with a new
diverter straight away could we try to clean out the old one?
re install and it work. no, didn't work at all.

new diverter installed, still not going fast enough to engage micro.

then, just to prove a point to my self after following the small copper
pipes to the brass valve where the blow off is and using man's
instructions to idenify parts(cant remember what it's called)i opened
the boiler drain off valve a little. then suddenly the boiler kicked
into life and supplied hot water(the hot tap was on all the while in the
bathroom)

could it have been air trapped in the small capilaries that active a
large diaphragm on the diverter, or is it just some coincidence?

how would you have diagnosed the problem, bearing in mind the
boiler/heating was fine.