Just thought I would reply to this topic with an explanation of why my gearbox was actually leaking. What had caused the leak was rust, it had crept up in between the tin plate and the rubber gasket/seal as when I pulled off the cover the was big flakes of rust between the mating surface of the rubber and the tin plate.
So I cleaned up the tin plate with a wire wheel on a bench grinder and just picked any rust off the rubber with my fingernail, and then put some Dirko gasket silicone on the tin plate and bolted it back on.
I have yet to put oil in the gearbox because I am waiting on the silicone to set so fingers crossed it doesn’t leak.