just started problem where does not fire up when turn on hw tap. sometimes if turn tap off and then turn back on it will or if i flick the central heating/ water or on off switches then turn on will be ok. or if i turn on central heating then hw is fine. does this sound like diverter valve or circuit board problem?
It sounds like neither diverter or circuit problem, simply sounds like a wearing diaphragm in the water section not lifting the servo valve to make the m3 microswitch.
Get the diaphragm changed before looking elsewhere.