The variable capacitor is clearly not going to be able to stand much power but at 20W it doesn't appear to be arcing. This capacitor has a reduction drive and I'm using a switch to add a bank so that I can tune both 40m and 20m. Joining the tuning shaft to a dowel rod with a piece of plastic pipe and cable ties worked out quite well although there's a little slippage.
Tuning is incredibly sharp but a very low SWR is achievable.
The offset of the tuning stub was a guess but seems to be reasonably close to 50 ohms. I had a pre-arranged contact with Stephen, VK2BLQ who could hear me reasonably well.
Reception seems excellent and of course the very narrow bandwidth leads to nice low noise.
Had a visit from Bob Bray who liked both the shed and the loop antenna, although I suspect he wasn't impressed with my impression of a circle.
Robert John Bray is a prominent astronomer who has been honoured by having a main belt asteroid named after him.
Update - motor drive tuning
I've changed capacitors and added a 36RPM motor drive and a box to switch direction. The drive is still way to fast and now I'm looking for a reduction gear or multi-turn capacitor of some sort.