Hi Les,
There is probably nothing wrong with your gearbox - the mainshaft has a felt seal that keeps back the majority of the oil, but the coupling has a spinner that throws any escaping oil into a trough and a gallery to the front layshaft bearing. This gallery also acts as a breather for the gearbox (as the oil level is well below the layshaft - the larger gears just dip into it). With the gearbox stood on it's front end, the oil simply runs in the reverse direction and escapes into the bellhousing. If you wash the clutch plate in petrol, it should be reusable.
Regards,
Eddie.