Martin and Fellow Listers,

If, like Martin, you get showered with pressurised transmission oil when
pulling the fill plug, you have another minor problem you need to address
before you before  you blow the mainseal on the transmission (which
requires removal of a major component, the engine, to replace).

There is a vent on the top of the transmission, which looks like a large
brass bolt, which is intended to let air pass through it while
discouraging the entry of dirt and water.  You need to remove this and
clean it out...  It's plugged.

Regards,
Jon Kanas