The message from geoff contains these words:
Sorry, I should have made it clearer. I have the Installation and Servicing Instructions so I know which connections are which (if I think to look at the right diagram) but my problem is more basic than that - I am having trouble work out for sure what I should expect to see on a meter at each stage of the process. Which process is not being helped by the fact that I seem to have recorded volts on white the wrong way round when looking at the boiler last night.
T1 is white and goes to GV2. T2 is black and goes to GV1. T3 is blue and is common to GV1 and GV2.
When calling for heat volts present all the time across T2/T3 so is that the pilot light valve which remains lit during main firing?
When the main burner is firing there are volts on T1/T3 and loss of volts turns the main burner out so is the gas valve is OK and I need to look elsewhere for the fault?
But where and what do I look for?