Thanks for all the replies...Most helpful. I've now lots to chew on...
Took off the fuse panel yesterday to have a look... Saw predicted rat's nest of wiring
... including a strange pot shaped metal thing with some of the battery
wires attached to it, as well as a box with "isolator" written on it... Have decided to get an electrician to look at it...
diagram that Maine
Sail and that other chap advocate makes sense to me, and I do think that that's the way to go... just that I need someone who's a little more brave around hybrid British/Thai (Moody lived in Phuket) wiring to do the overhaul