Had to go and sit in the cab to check this Tim. Memory for such detail definitely not reliable after 30 years park up...
So, sitting in the cab, the left side (passenger for the rhd vehicle) mirror view sits perfectly and unobscured in the small wing mirror. The driver's side mirror is viewable through the larger wind down window. In my opinion they both give good enough visibility on each side of the vehicle.
I agree with you that they are necessary. My '38 sedan never had these side mirrors and 40 years ago I don't remember it being a problem. After driving modern, well equipped vehicles with good visibility I am certainly on the lookout for some suitable door hung side mirrors for this car.
Am sure you will solve this one. The picture of the two Brits together in your shed looks really nice. They complement each other well.