Got the *astard! (Richard, take a bow).
But here's the thing. The leak turned out to be at the banjo/pipe solder joint, not the actual banjo / oil tank union itself.
After reassembling the thing 3 times trying various types of sealing washer, I pulled it all out and pressurised the banjo and pipe with air and so found the leak. I desoldered it, cleaned it up, annealed it and resoldered it using silver solder. Job done.
Now for the last leak... Yes, that gearbox/primary case drip. I've tried to fix this one before, carefully aligning the sliding plate etc, but no good.
I believe there's an oil seal conversion for this joint? Anyone got a fix for this that doesn't cost a wad of money?
-Jools