Hi guys, thanks for your help. I think I have resolved the issues. There are no marks on the cams, but the Norton Owners club provided me with the answer for the valve timing:
Turn the engine to top dead centre, if you have the mag timed up make sure you are on the stroke with the points closed. Turn the engine forward so the piston moves down a 1/4 inch and fit the exhaust cam so it has just closed, checking the rotation as above. Next, turn the engine back over TDC so the piston is coming up 1/4 inch from the top and set the inlet cam so it is just about to open, again checking the correct cam rotation.
You might not be able to get the positions dead accurate but you should be fairly close with a fraction more measurement rather than less. The early engines have a 3 key-way half-time pinion with the slots in different attitudes to the teeth and this allows finer setting.
I have sealed the cap (no more dribbling!) and heavily greased the valve stems which seems to be helping the engine run cooler (doesn't seem to be getting too hot? The bike now starts easily and runs cleanly and cooler!
Thanks again for your help - what a great couple of clubs!
Gary