The immersion heater is stand alone, and is put on say an hour before we
need the bath. I normally turn up the boiler, and turn down the central
heating to give some extra hot water

I'm still not sure what sort of system you have. Presumably you have a
cylinder with an immersion heater in it. Is the boiler plumbed into this at
all? What do you mean by "turn up the boiler"?

Christian.