Just a bit of an off the wall one for you here Andy, but over the last 11 years, my Jatco would exhibit similar symptoms, but they would be when VCU bearings had gone or the VCU was stiff. Three separate occasions over the years. Each time I was convinced it was duff oil or maybe a solenoid...
Slightly off topic but related, saw this professional effort at a rear guard on a facebook group. Sadly the person who fitted it didn't answer questions about where they got it from or how they fabricated it.
I'm sure you're on it given your obvious knowledge and your comment about the fluid level, but I would say check, check and check again the fluid level, engine warm, running, cycle though gears etc. Next would be the selector components I guess.
Well today I have had opportunity to add a post to this thread following a few days of posts of jobs being done by a learned friend on my car.
Assisted with the fitting of a poorly manufactured fuel tank cradle which needed considerable modification, and the fitting of a refurbed subframe and...
Might be worth getting it shot blasted and painted. You may make a bit on it. I paid £100 for a second hand FL1 one and it was blasted, zinc primed and sprayed for £80
I have to say, there is some stirling work going on on the FL1 @Nodge68. That subframe looks pretty solid for the most part!
*edit*
P.S. nice shiny new jack there!