Fluctuating water temperature in shower

Could someone please suggest what might be causing the temperature of the water when showering to fluctuate from slightly too hot to slightly too cool repeatedly every couple of minutes or so. Reading through a variety of forums suggests that it could be either a faulty pump speed control board or a scaled up plate heat exchanger and given that these are both fairly expensive parts, I do not wish to replace either unneccesarily. Additionally, neither the green light nor the red light are illuminated on the front of the boiler although the red flashing light on the control board in the top right corner appears to be functioning correctly. Any guidance would be vey gratefully received. Thanks!

Does the hot water do the same at the taps?
If they are ok sounds like shower.
Is it thermostatic?
Mike

Hi Mike - apologies for the delay in my response, I have been away. My shower is a basic mixer tap model - not thermostatic. The problem does occur at the other taps but does settle down to a more constant temperature after several hot/cold cycles. I have lived in this property for a couple of years, Monday to Friday - returning to my family home at weekends. I have been in the habit of turning the boiler off at the timer over the weekends and wonder whether this has contributed to the current hot water situation? Thanks for taking the time to respond to my enquiries.

Hi
The first thing I would do is change the DHW heat sensor. This is a classic indicacator of it failing. If the problem persists after changing it I would then change the phe return sensor too and check the heat exchanger for scale contaminination.

Regards

Gary