Oil leaking into primary drive

There is a seal behind the drive sprocket. , Once the engine is drained and then used regularly the problem may either go away or be minimised. Its quite likely that the seal is old and would benefit from being changed. This can be done without a major stripdown ,but you may have fun removing the engine sprocket which will need a good puller. A couple of short self tapping screws and pincers/claw hammer will usually get the seal out.

... is long enough for all the oil in the tank to have migrated to the crankcase ("wet sumping") and if the seal mentioned by Robert is at all leaky, from thence to the chaincase. I would drain the crankcase (and chuck the oil away as it'll be well past its use-by date), refill the tank with the correct oil and see how it goes.

Most Nortons wet sump to some extent so if it's not used more than once a week or so you may want to think about a non-return valve although opinions are divided on these.

... to open up the primary chaincase as the oil will probably have soaked the clutch plates. However, if it's held the 750ml successfully you may not want to risk getting it to hold oil again.

The oil level in the case should be just below the level plug. I put the bike on the side stand and let any excess drain off. If you have a non leaking case (a bit unusual!) the level will not change over a week.