Julian,
It is possible to flood the engine with petrol, but the amount of extra fluid that you indicate was in the oil tank seems more to be more than would be drawn in with normal kicking. One possibility is that one or both carb float valves are sticking open allowing petrol to run into the inlet manifold, although this will normally manifest itself in a very obvious overflow from the carb tickler breather. How much kicking did you have to do to get it fired up? and once running was it running properly.
On another point did you innitially check the oil level while the bike was running? Most experienced Norton owners never fill the tank up to the maximum on the dipstick as this is actually too high and leads to leaks from all sorts of places, the general practice is to run the level between minimum and maximum on the dipstick while the engine is running.
My inclination would be to make sure that the bike starts and runs reliably (starting within about 5 kicks even when cold), not running too rich etc. Then drain the oil tank, sump plug and filter if fitted. refill with fresh oil, set the level while running at normal operating temperature then moniter the level after every ride.
Good luck
Dave