There are two anti drain valves in the engine which need to be checked. The first is in the timing cover...you will see a small (3/16" Whit) hex. headed fitting just below the BSA logo cast into the cover. Remove this and underneath you will (should) find a small spring...behind that is a 1/4" diameter ball bearing. After removing the fitting and spring put a parallel punch down the hole until it contacts the ball...then using a light hammer give the punch a sharp but not too heavy tap. This will re seat the ball into the casing. I would recommend replacing the spring with a new one...available from C&D Autos. Replace the spring and fitting and wait to see if the situation is improved.
If it is not you will then have to remove the sump plate and gauze filter....The second valve is located in the bottom of the oil pump.
This one relies on gravity to seat it into the bottom of the small tube that protrudes from the lower plate of the pump. The ball is retained by a small wire pin.(above it in the tube)
If any debris is preventing the ball seating correctly it will leak.
Using a scriber or similar lift the ball off its seat (by pushing it back up the tube with the scriber). Then using a thin piece of wire with the end turned through 90 degrees run around the 'seat' as best you can...it's a fiddly job. The end of the tube is 'turned in' to form the seat on which the ball sits.
Allow the ball to drop after you have done this, clean away any surplus oil and leave it for a while to determine whether it is seating correctly. If no oil appears on the bottom of the tube replace the gauze and plate.
Both of these valves are 'downstream'from the oilpump and though they will have more to do if the pump is worn they should reduce leakage if functioning correctly.
Personally I would not recommend using aftermarket anti drain valves for a number of reasons. A tap in the oil feed line is OK if it earths the mag but if it doesn't you WILL forget it one day or another....Ian
email (option): julie@wright52.plus,com
Hi Ian, just to follow up on your last comment, talking to a fellow biker friend last week and asked how his Norton Commando was, "not too good, had wet sumping problem, dropped in a inline tap and forgot to turn on, had only been ticking over in garage when I remembered but too late, now rebuild time", ouch!!! 
email (option): ianhince@tiscali.co.uk
Ouch, and you can't put a mag cut off switch on a Commando With a Commando it is just a case of lapping in the oil pump. Mine does it if left a while, does it really do any harm to just start it? It smokes for a few seconds then it's ok
email (option): horror@blueyonder.co.uk
There's not really much more you can do with a 16H, there's not even a sump drain plug.
email (option): horror@blueyonder.co.uk
Yes there is Dave! Bottom rear corner on timing side. Brass hex plug.
If you leave your bike standing for any length of time, you will undoubtedly need it. Ron
email (option): ronpier@talk21.com
Ah there it is, I thought it was odd without one, and the sump guard covers all the bottom of the engine.
Cheers Ron
email (option): horror@blueyonder.co.uk
i found a way to stop wet sumping use 50 weight oil i tried to pore some yesterday it was like treacle so it would take at least a month to reach the bottom of the engine
The trouble is Roger, this weather it will also take about 2 miles before it reaches the bearings
Ron
email (option): ronpier@talk21.com
A 40 weight would be more suitable for the average British weather conditions...keep the 50 for when (if) we have a really hot summer...
...ian
email (option): julie@wright52.plus,com