Not sure you can not sure what the lowest on that stat is.Just set the heating on constant (not timed) and set your thermostat to say, 10 degrees.
You need to set the stats frost setting to 5deg and then switch it to 'OFF' That way the stat will only call for heat when it reaches 5 deg. If the boiler is still firing up then it can only be the boilers internal frost setting that's doing it and that cant be changed. try a thermometer next to the boiler and see how cold it is in the location of the boiler.