Thank you powell30 for a fantastic quick response. Will try the thermistor first as it should be easier. Assume you mean the HW sensor.
Many thanks again.
Have a Baxi 105e and had venturi and hot water valve replaced recently by engineer. He suggested turning flow down to minimum (admittingly he had other jobs on).
Get about 2/3 litres of very hot water, then the water is only warm. Reducing flow helps slightly but still not hot. CH okay, in fact...