I have a back boiler with a thermostat on the top of it, scaled from 1-6. I
also have a room thermostat which just controls when the heating works as
far as I can see.

My question is what should I have the boiler thermostat set on? It is now on
'4' but is it more efficient to have it set on 5 or 6?

The hot water temperature seems ok, slightly too hot to hold your hand under
for long but I assume thats ok.

I am new to all this, as this is my first house so go slowly

Thanks,

Phil