One more thing you could try.
IIRC there is a rubber cover low down on the nearside of the gearbox bell housing to get to the clutch bleed nipple. You can get to it from under the car.
If you take that out you might be able to see the slave cylinder with a mirror or even better a camera borescope.
You might be able to see if the fluid is leaking from the slave cylinder.
Just a thought.
