when i turn on hot tap sometimes the boiler ignites other times it attempts to light but there is just the occassional clicking. but on heat/water mode it lights could it be the diverter valve?
Yes
Or and, the HW flow sensor, probably attached to the divertor valve, Some are micro switch, try and see if the arm moves to operate, if yes, check the switch 1st. Lot cheaper than a divertor
