Stephen, “..So I’ll just to have to stick with a set off car jump leads if the starter battery fails. The only issue with that is the need to lift the saloon floor and then take of the lid of the battery box – not something I would relish in even a moderate sea.”
So why not install a set of those car jump leads – screw them to the battery terminals properly, not just with croc clips! – and then route them to any convenient place where you could fix a heavy-duty changeover (or rather “interconnect”) switch? ..You’d only need to use positive cables, as you could connect the negative terminals – of both batteries – together permanently.
Choose any convenient place for the heavy-duty “interconnect” switch, and then if the “starter” battery’s flat, just twist the switch to connect the house batts to it, and then switch back to “not connected” when the engine fires.
That’d surely be far better than lifting “..the saloon floor and then take of the lid of the battery box – not something I would relish in even a moderate sea.”