I agree that it is probably a leak from the crankshaft, but do be aware that the splined hub of the Centa coupling has a seal on the outside between it and the crank case, but relies on a bit of mastic between it and the centre race of the main bearing; this is the most likely source of your drip. I have upgraded mine by boring a recess in the hub for an "0" ring. This is worthwhile but only if you have the engine out for some other reason.
It will not be gearbox output shaft because the clutch is next in line after the Centa bell housing, not the gearbox.
Monitor the drips and if it does not get worse wait and sort it when you need a new clutch or Centa coupling inserts.
Good luck,
Phil
