There is a potentiometer on the control board that can be used to
limit the burner gas pressure.

If there is no thermostat for the outgoing water you should fit a
thermostatic mixing valve on the water outlet. It might provoke more cycling
on the boiler, though, if the (presumably) modulating burner won't go low
enough. Alternatively, turn the tap on higher.

I wouldn't play with the gas controls.

Christian.