I can confirm that I have no more oil leaks!!!!
It was the gallery plug.
Big thanks to everyone who has helped out on this with tips and ideas, helping me consider all the alternatives and giving me confidence I was doing at least some things right.
Stupid mistake to make nonetheless - but one of those er, learning experiences that means I'll never miss another dodgy oil gallery plug as long as I live.
Pressing on with finishing things off - though top of the list is to actually run the motor for a decent time and get the carbs dialed in etc - so it's entirely possible there will have to be some re jetting etc there before I'm ready to roll.
And then I need to actually run it down a road and check the gears and brakes work (so far I've driven it about 3 feet - the free length of the lockup). Opportunity for more gremlins to emerge.
I'd love to say VW Action was a definite but work and other commitments are going to eat into my time over the next 2-3 weeks. 27th is my last pre-Action chance to test at Crail, so I'll just keep moving forward and see where I get to ...
Thanks again everyone.
Stewart