It was at this stage that I chickened out and decided not to weld the parts together
The bolts were holding the pieces together very securely, and it just didn't seem worth taking the chance of ruining them, although a really nice tig weld would have looked nicer.
To mount the mirrors on the end of the arms, I needed to tap a thread for the steel ball mounts. They have a very coarse thread, and after some head scratching, googling and counting threads per inch, I decided this must be 1/4 UNC. £4.45 delivered, and a few days wait and I had my first cut tap, drilled the holes and tapped the threads and the mounts fitted OK
Next came quite a lot of smoothing, shaping and polishing. I tried to make them a little lighter and look at least a little bit aerodynamic. I considered drilling lightening holes, but decided trying to clean those would be a nightmare.
Here is a photo of the polished parts:
And here assembled:
This is the finished mount with mirror attached: