Nigel wrote:

I do know that if I remove the black cable joining the
2 switched negatives the domestic system still works ok but the batteries
don't charge. I assume from this that the positive cables must be
connected elsewhere
I'll investigate further today,

Leaving aside the issue of whether the engine and alternator are
positive or negative ground, the alternator will have two output
connections, one through ground and one through an insulated
terminal; one of these will be positive, the other negative; for
the rest of this it doesn't really matter which is which, so long
as you can identify them.

Chances are you'll find that the minus output of the alternator
goes via a single cable to the switched minus of the engine battery
(and then via your mysterious short link to the switched minus of
the domestic, and that the plus output of the alternator goes via
two separate cables to the two switched plus terminals. I have a
feeling it might be those black cables with the red tapes around
their ends.

Leave the short link in place.
Remove the plus alternator feed cable from the switched domestic,
either disconnect both ends of the cable and throw it away, or
double it up so both cables go to the engine switched plus.
Then connect your VSR between the two switched + terminals.