Oh, ye of little faith (including me!)....
Low ratio, first gear, diff-lock engaged, car tyres at normal pressure*, trailer tyres almost flat, outboard off.
The traction control didn't even flicker, unlike my heart as I headed back!
Not sure I'd want to try it with a heavier boat.
* local advice was that is not usually necessary to lower the pressures but that its the trailer that'll get you - the trailer certainly made the deepest ruts!
I'm thinking of changing my signature to "Blue car. Blue boat. Brown trousers."