Just tried to drain some out. I got more up my sleeve than in container. Will leave it 'til I got the new part and then get stuck in.
I think this may have been on the way out for a little while, just came a bit of a conclusion this afternoon. There's always been a certain amount of guff on/around the top of the gearbox, I assumed it was oil vapour from previous split breather hoses or oil sprayed up from the diff seals which I already had replaced.
I've never removed the air box to have a good look around that bit, but can see more clearly now what's happened and where and on a positive note, I also saw how easy the TT selector change is!