One other thing - the whole setup seems to be powered via the cable
from the boiler (via a permanent live), there is no direct mains into
the wiring box. Is this OK to do? There is a spare outlet in the
cupboard which is currently unused (maybe originally for a second
immersion?) so I could power everything from there if that's preferable
- in which case presumably leave the perm live from boiler unconnected?

I would have thought all CH power should be taken from one place, so if
there was cause for you or anyone else, to be doing maintenance, the whole
lot can be turned off from a central place, thus minimising the risk of
forgetting to turn the part you are working on, off.

As for the other questions, I am not sure, however I will be interested in
the answers as I need to rewire my system at some point, as the plumber who
installed it, for some reason best known to himself, wired it so the HW has
to be on for the CH to work (Fully pumped system with a three port valve, so
I cant see why this is the case!)

Sparks...