The pump overrun on the Potterton 80e is thermostatically operated. It will only run if the temperature of the water is in excess of 80 degrees C.
So, from what you say it may be faulty. When the boiler is set at a low temperature, ie less than 80 degrees, all is well, yet when you run it at a higher setting the overheat trips which suggests the overrun is not operating as it should.
So, from what you say it may be faulty. When the boiler is set at a low temperature, ie less than 80 degrees, all is well, yet when you run it at a higher setting the overheat trips which suggests the overrun is not operating as it should.