I was wondering you could blow gently into the oil vent, and see if you can re-create the bubbles.   

I tried that. No bubbles, and no the breather is clear + the dipstick wasn't in its hole.

 

Is there any chance you sealed off an area of the bottom of the engine with the mounting plate, and are trapping drain oil?   

That's what I'm beginning to think must be happening. If no-one has any other suggestions I guess I'll take it apart tomorrow. When I get the mount plate out I'll drop some oil through the returns and make sure there are holes in the mount plate under these points.

John