Scalding hot water from shower?

I fitted a new aquatronic 1 shower last week and commisioned it as per the instructions.

It has worked fine until this morning when the water ran scaldingly hot. The heat gauge was set to 6.

I turned the temperature gauge to about 2 but it still ran excessively hot. I was just about to turn the unit off when the pressure relief device burst.

I suspect that the fault is with the water heater or thermostat.

Would this cause the PRB to activate

Thanks