The propane burner was fixed by RTFM. There I noted that the burner was intended to be run straight off the bottle with no regulator.
I replaced the jet with one of the original size, and connected the supply hose directly to the bottle without a regulator. Resulting in a beautiful clear blue flame. Which did not show up well on a photo so you will just have to take my word for that. I intend to experiment with slightly larger jet openings to fine tune it.
Then to finish the workshop session I made some fiddly little bits for the fire door. “Bling” as my friend John would characterise it.