Does anyone know whether the rear low side panels on the XJS simply unbolt like the XJ6 or if they are spot welded?

I can't really tell without taking the side bumpers off and the fit/finish is a lot higher than the XJ6, so it could just be well hidden.

 

SS
If you mean the panels below the rear side bumpers that go down and are curved round to the exhaust tunnel, they are welded.

 

Are they just spot welded along the seam that sticks out?

 

As this pic shows, there is a folded seam and it is welded along this join, but I cannot remember for sure if it is spot or seam welded, but I would be amazed if it were not spot welded, as everything else is on the car. But that piece of bodywork is also welded into the rear of the wheelarch and the tunnel vertical at the bottom, and the rear under the light cluster, which are the difficult places.
I had to patch and weld up holes along and below that seam, but luckily my poor-looking welds are largely covered by the bumper. But I did go to enormous lengths to protect the metal afterwards.

 

Nice job greg