Euro combi - Red light keeps coming on and cutting off boiler?

I have a Euro combi boiler and the red light keeps coming on and cutting off when in central heating mode and the water is heated up. When I press the button below the red light it goes out and the process starts all over again. I would also like to have instant hot water when I turn on the tap instead of waiting for it to heat up.

I have had the internals cleaned and the pressure appears to be fine, but the boiler man cannot determine if it is the thermostat or the pcb (electronics board).

The thermostat does what it is meant to do i.e. cut off boiler when water gets too hot 80 deg plus; could there be another problem with it? The electonics board is about 250GBP and the thermostat is only 30GBP but I do not want to give this seemingly incompetent engineer any more money than I have to.

Getting a new engineer would cost more as there is that annoying initial call out charge to confim what I already know; the boiler is sh*t, it is not working and it might be the thermostat, plus all the work that may need to be done.