Christian McArdle wrote:
At present we need both the immersion and the boiler to give us a
decent bathful of hot water.

Do you mean that your cylinder is heated from the boiler anyway, and
that you use the immersion as well to get that little bit of extra
temperature to fill the bath before it goes cold?

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

If so, the problem is that your cylinder is simply undersized for your
purpose and should be replaced by a bigger model. If replacing and
your mains pressure and flow is good, you may want to consider
replacing with a heat bank or unvented cylinder, which will fill
baths in no time at all and provide a superlative shower.

Mains pressure flow is excellent; however storage space for a bigger
cylinder is not really an option. As stated in a previous thread, I have a
plumber arriving tonight to inspect the system and make some
recommendations.
Thanks for your help,
Regards
John