Breather question

The oil tank has the breather outlet at the top  (not the pipe welded to the filler cap spout) that should be piped to the back of the ham cam. Try and get the one way valve as close to the engine as possible.  

Mk3 breather goes from back of timing cover to oil tank at filler neck. As Peter says, put your valve in this line. This is a single run, no tees.

Vent from top of oil tank goes to a black oil separator tower behind the airbox from which two pipes exit. The large diameter, goes into the back of the airbox. The small one goes to a tee in the link pipe between the inlet manifolds.

See https://andover-norton.co.uk/en/shop-drawing/879/oil-tank-and-fittings-oils-lines-cartridge-filter

You can probably dispense with the oil separator and go direct from oil tank to airbox.

Do away with the separator and run it directly to the air filter, if you keep the oil level around the minimum mark and use a decent semi or full synthetic oil you should not have any issues with condensate. 
If you fit a one way valve then make sure that you avoid short rides as they cause corrosion in the engine, other than that they work fine. 

The assertion that breather reed valves cause corrosion is questionable.