Baxi 80E Combi.
When boiler switched to hot water only, call for hot water works fine.
When boiler switched to Central Heating & Hot Water position, and hot water is turned on the diverter valve opens both hot water & central heating (circulation pump switches on) for the duration of hot water running.
Have changed the PCB, Diverter Valve, DHW Thermistor temperature sensor & Safety thermostat.
Have followed same sequence with room stat on & off, and also with timer isolated, all with the same result.
Someone suggested the Plate Heat Exchanger, which I removed, looked OK, and refitted, couldn't see how it would correct my problem!
Any ideas would be much appreciated.