Nigel wrote:

I have a 24v system comprising 2 battery banks 1st (2x12v) engine and the
2nd (2x12v) domestic, each battery is approximately 100ah (ie 200 ah per
bank).

That's only 100Ah per bank, because within each bank the batteries
are in series. Two 12V 100Ah batteries in series make one 24V 100Ah
battery.

The banks have 2 separate double pole switches, until now in order
to charge the domestic batteries there has been a link between the 2
negative poles on the switched side of the switches. I brought a VSR
(voltage sensitive relay) to replace this link, but when I came to fit it
I found that it needs to connect between the 2 positives of the battery
banks. So I plan to buy another relay (an ordinary one), wire it in the
link currently joining the 2 banks, and then, use my VSR to switch this
relay on and off. Does that sound sensible or have I lost the plot.

I think quite possibly you may have. I don't understand your description.
If your ASCII art isn't up to drawing a diagram, perhaps you could give
us a detailed "thigh bone's connected to the hip bone"-style description
of your circuit.

Let's say your engine bank has two batteries E1 and E2, and your domestic
bank has D1 and D2. Let's say E1- is permanently connected to E2+, and
likewise D1- to D2+. I'd normally have expected E2- and D2- and the
engine block to be permanently wired together, and any switching to
involve E1+ and D1+ and the starter+ and domestic distribution+ and
the alternator+, while starter- distribution- and alternator- all being
permanently connected to the engine block too. How are your switches
and the link connected exactly?