Level was on the min mark (warm , through the gears etc) and all was fine. No leaks or anything.
Cheerfully 'added 1 pint' (as advised by dipstick). Left it overnight. Loads of drips on the floor.
. So at the 'min' mark - no drips, at the full mark - drips.Clearly something other than the pan-gasket was leaking. Hot suspect is that vertical shaft pin thing. Given that I have no intention of dropping the gearbox out of the thing until winter, and the Nats etc are coming up my new strategy is to just leave a drip pan under it to catch the half dozen or so nightly drips and just enjoy the car.
Still, at least I have a sump plug now, so not entirely wasted.