Ok so, I had the sprocket cover off and everything behind there is fine, brake cleanered all the gunky crap off and its totally bone dry not a hint of leaking oil from any of the seals. So thats another potential ticked off.
Whilst I was down there, I checked this again.
Ive had the bike on the centre stand for a few days now, and its still leaking from exactly the same spot. Blew the oil away with brake cleaner and kitchen roll, and you can see it slowly build up again in the recess of that allen bolt, cleaned it away again to make sure, and sure enough it builds back up again. So Im now 99% sure this is the culprit. Now to find out *why*
Theres no obvious damage to the bolt or surrounding case. One of my friends suggested it might be missing a washer, but theres 14 bolts holding the sump on and none of them have washers, I refuse to accept that even the most amatuer of home mechanics who owned it before me could have possibly forgot to put all 14 back on... Does anyone know if theres meant to be washers there?
So my next idea is to take that bolt out and see if it pisses out oil everywhere... and if not, to PTFE tape the bolt and see if that seals it up.
Ive had a measuring jug underneath the bike catching the leak for a few days too, seems to be losing somewhere in the region of 50-100ml overnight, assuming thats constant when its parked up at work too Im looking at around worst case scenario of 200ml of oil loss a day, that seems like a lot to be coming from one tiny little bolt.