You have my commiserations about your boiler-
Ours has had a mysterious intermittent fault since November 25th!!
It goes off (shuts off) and cannot be restarted between round about 5pm (give or take half an hour) and 9pm (give or take half an hour)
During this time,we have no heating,and no hot water!
We had our routine British Gas service after this had happened once.Passed its test with flying colours.
Then it started doing it every night!!
They came back out (but not when it was off) and could find nothng wrong with it..
Then Insurance company sent their men -again,not during the "dodgy" hours.
They found nothing wrong with it,but replaced various parts just in case,however,this made no difference whatsoever.
Got the British Gas to check in coming gas pressures-which was perfect-including during the "dodgy" hours.
Everyone is baffled...
The only thing that we can think of,is that somewhere someone has a faulty bit of electrical equipment,which they use between those hours.
Usually,this would cause the trip switch to go off,and then it would prove the electrical theory-but unfortunatley,we have the old fashioned fuse box,with no trip switch
Insurance company say they will not come out again,as we have a "functioning boiler"
Quite how they came to this conclusion I dont know,unless they think that central heating (and hot water!) boilers are supposed to go off for 4 hours every evening! Aaaaaargh!