"Christian McArdle" wrote

Why the immersion? If you have gas heating, the immersion should be an
emergency backup only. It costs around 4 times as much to use. If the
thermostat is set higher in temp than the gas (or lower in the cylinder),
it
will even use it in preference to the cheaper source.

because I want the hot water cylinder to heat back up again after use, as
quickly as possible.
The immersion is set to 40 deg C, and the cylinder stat to 50, so the
immersion only kicks in if the cylinder is drawn on heavily. As a result it
only takes about 10mins for the water to be ready for the next showerer.

Neil