Mark,
I would suggest that, at this point, it might be a good idea to remove the non-return valve and carefully check it out. Inside the body there should be a very thin disc that is free to float between the 2 seats - that is, there shouldn't be a spring to keep it against the seat. One seat should have 4 slots to allow the oil to pass - the other seat (without slots) prevents oil from returning. Make sure you have flow in one direction only, i.e. into the engine.
With regard to the extra fitting on the top of the crankcase - there is usually a tap fitted here so that it is easy to prime the crankcase with extra oil should it be found necessary. The tap should be turned off when running. Some later engines just have a blanking plug instead of the tap. As you have a blanking plug in the timing chest, instead of a pipe fitting, maybe this and the one in the top of the crankcase have been swapped at some time, this could possibly be the cause of your oiling problems. If the timing chest cannot vent, then the crankcase wont either, and crankcase pressure will build up, preventing oil from draining from the tank.
Regards,
Eddie.