In the op's position, I would replace the CU with a larger split load
model and add some extra circuits. If it's easy to do so, I would
find out what was and wasn't "ring" on the downstairs 32A breaker, and
if it looks like a radial circuit has been added to it, move this onto
it's own 20A breaker.

Alternatively, depending how much load the OP expects to place on the
conservatory circuit, he could use the esisting breaker marked
Immersion heater to feed it as a radial rather than a ring, and
either;

1. Forget about immersion heating.

2. (Getting non complient here) put the immersion heater on an fcu on
the upstairs ring (assuming the cylinder is upstairs).

3. (getting even further away from compliance perhaps) leave it on the
same breaker as the new radial. As long as he doesn't put a 3KW
heater on the radial at the same time as the immersion is on, it
shouldn't matter.