Fixed all the dashboard gauges today.
None of them (oil pressure, temp gauge, volt meter, tilt/trim, fuel gauge) were pointing where they should have been pointing for the given input resistances from respective sender units, in fact they were all pointing completely off scale.
Fixed by simply using the 2 spoons method to remove the dials and refit in the correct position (but not before I completely dismantled the dash gauge cluster thinking it seemed I needed to use too much force on the spoons and would break some gauges so investigated if it was possible to use another method). When it seemed it was the 2 spoons method or nothing, try to fix it and not loose much if I broke gauges anyway, I went for it and used the 2 spoons method and all the dials popped off OK. If I'd broken some gauges I'd have needed to fit replacement gauges and fabricate a new dashboard (maybe from teak etc), so I'm glad the fix worked. Correct readings confirmed by temporarily wiring resistors in place of the sender units.
All gauges point to the correct position now
Confirming coolant temp sensor response / linearity
All gauge responses (relative to given resistance input) seem correct according to tables like this
Someone must have had the needles removed at some point in the past and not put them back on in the correct position.
The ex owner told me he knew nothing about engines but electronics was his thing. I hope electronics wasn't his only thing lol because I reckon he rewired the radio and fitted the sub (which I'll probably remove because its cabinet protrudes into the cabin). I only touched one of the scotch locks they'd used after turning the radio on and it worked. I hate scotch locks, especially in a marine environment.
Next I want to remove the makeshift immobiliser that's fitted. An immobiliser problem on land is a pita, at sea it can be a safety concern. And besides, anyone with basic knowledge can start any carb engine immobiliser or not.