Had a few hours to myself this afternoon so I took a deep breath and started pulling the engine down.
With the head off the piston crown was black but fairly dry.
Pulled the cylinder off and the piston skirt was thick with oil - on the exhaust side of the piston skirt the oil was black and smelly, pulling the cylinder off the bottom of the liner was covered in thick oil, also a fair bit of black smelly oil in the exhaust port.
Piston off - gudgeon pin had a thick layer of black smelly oil in the bore.
Turning the crank through 180 degrees you could clearly see that the flywheel had been sitting in oil.
Draining the gearbox oil I was about 100ml short, it looks like the 100ml must have found it's way into the crankcase. I did remove the screw from the primary case so the off 100ml is not in there.
Undoing the screws that hold the crankcase halfs together, most of them are covered in horrible smelly black looking oil, I also noticed that there was signs of leaking between the crankcase halfs, so I am pretty sure the seal between the crankcases has broken down, or worst case their is a crack somewhere.
I'll be splitting the cases in the comming weeks.
What amazes me is that the bike did not really smoke that much on the rare occassions it actually ran.
Next problem is removing the gearbox sprocket nut - it must have been done up by a gorilla, despite heating, and banging the socket tommy bar with a mallet the thing will not move, started trying to split it with hammer and chisel but thinking is that you really dont want to be hammering on the shaft - time to break out the dremmel.
Guess this is progress...