Christian McArdle wrote:
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.
That's right Christian.

Is the boiler plumbed into this at all?
No, it's purely heated by the electric supply

What do you mean by "turn up the boiler"?
We have a glowworm Ultimate 50 BF boiler that is used with a Potterton
timer. The boiler is on constantly, albeit at a lowish setting. There is a
Honeywell thermostat in the house. If I turn up the thermostat to say 20
degrees, then the boiler kicks in and provides the central heating. If the
house is warm enough, then the boiler ticks over . If I turn the boiler up
full, and turn the thermostat right down, that gives us the extra top up
when using the hot water.
Hope this makes sense.
Thanks again for your help,
John