Cheers Silver', sorry for the late reply . . .
Didn't go with a 400 (mine's still in bits), I went with a friend and his 1976 SOHC 810 Honda.
We were supposed to take his 1982 Kerby Honda 500 Single, which I'll be riding at the Belgian Classic TT next weekend, but it had a Clutch problem on the Dyno beforehand.
Perfect day for it, pity it had an oil leak that stopped play :'(
Went out in the first session, and was impressed with how well it went.
I've never ridden it before, and hadn't been on track for a few years, so took it fairly gently (luckily).
First three laps were 'sighting' laps, following a staff member.
Just after he pulled off, I felt my left Boot slipping on the peg, looked down to see Oil all over it.
Pulled in to check it over.
Looked to be coming from the bottom of the gear-change cover, so we whipped it off and found the gasket was broken at the rear lower corner.
As we didn't have a spare, we refitted it with some blue Hylomar, wiped the oil off the rear tyre, and I went out again.
Kept an eye on it, and after four laps or so it was doing it again.
Pulled in and whipped the Crank cover off.
Gasket was fine, as was the surface of the Crankcase.
Then I looked at the inside of the Cover itself, and there was the cause.
It's not got an Alternator fitted, and the gap in the casing where there should be a rubber bung (where the loom passes through), was just empty, no wonder it leaked!
Booked in again for Monday 18th now, Kerby's just about ready to go.