Just back from the boat.
Access to the cabling behind the master battery switch and feed to the starter motor on a 321 is virtually impossible without dismantling parts of the boat under the nav station.
As a result trying to put in a big switch would not be an option for myself.
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.
Thanks for all the advice.