Laurie,
There is no seal behind the coupling on the front of the Mark gearbox. There is just an annular groove to collect the oil that comes through the bearing. At the bottom of the groove there is a drilling that feeds that oil to the front layshaft bearing. It is important that you only use an open bearing on the front of the layshaft, otherwise the oil will have nowhere to go, and will eventually spill from behind the coupling.
The coupling is on a large taper and is often difficult to shift. If you have the front plate off the gearbox, put a reasonable pressure on the puller, and then try giving the coupling repeated squeezes in the vice eventually, it will give up and break free.
Hope this helps,
Regards,
Eddie.