OK cup of tea time, I've done about all I can today (gotta see the girlfriend at some point to keep up appearances
).
The scottoiler is very likely involved - when I went to trace to vacuum hose, it fell right out! Unfortunately that made tracing where it came from a bit difficult, but it looks like there is an L-piece sticking out of the front cylinder's inlet pipe (not the top cylinder, which has the white plug). I didn't quite get to the point of removing the carbs so haven't had a good look at both pipes yet.
I've got the carbs with the heaters, so I'm guessing that when I unbolt the oil inlet & outlet I should expect a bit of leakage, however I'm also not sure if the heater is currently on or off - the tap is currently in line with the main oil line between the engine and cooler, so at a I guess I'd say it's off? Although I guess it is a moot point as there wont be a whole lot of oil in that part of system while its not running.
With the faff involved to get the air & battery box off (which has an elephant on it, btw), I'm thinking I might take the opportunity to get the starter circuit upgrade done (+ replace the starter brushes/terminal). I'll see how I go with funds.
BTW - any tips or things to do while I've got the carbs off?