I'm delighted to report I've traced my oil leak back to the VANOS, it's probably quite a straightforward fix (famous last words?). I thought it was my valve cover gasket and replaced that (and stripped head threads), still leaked but gasket looked like it might be seated incorrectly, removed valve cover again, realigned gasket, clamped down (no stripped threads this time), still leaked . So today I got brutal with the rags, brake cleaner and degreaser and cleaned the area at the front of the engine up like it was factory new. Sure enough it seems the leak is higher than the gasket and coming from the back of the VANOS unit. This is good news because only 3 o-rings seal that area, the ones that are being used sat on the shelf for a while and I need to replace them when I swap my VANOS unit back in anyway. The leak is small, unlikely to get worse, in a 'safe' place and will be fixed shortly. Knowing this gives me confidence to drive it like I stole it and I haven't had that feeling for some time now. Sweet.
Edit: something I did learn/confirm through this exercise is that 7nm is sufficient torque to lock down the valve cover. 8nm as documented in various web procedures and the BMW TIS stripped threads for me and is not recommended (by me).