Ok initially reported the transmission fault message appearing in the instrument display, but no codes in the TCM, link to post here;

https://www.fullfatrr.com/forum/topic74586.html

So have had the transmission fault message appear a couple more times, under load (pulling a horse box) or heavy acceleration. Gearbox seems to go into failsafe mode until engine switched on and off again.
We now have codes;

ECM U402 Invalid data received from transmission control module
TCM P0741 Torque converter clutch solenoid- circuit performance or stuck off

I am being advised I need a full rebuild. I would welcome any comments on potential courses of action before this step. The transmission has had a recent filter and oil change with genuine ZF. Old oil did not look too bad. All the best
Chris

Others;
1966 Jaguar 3.4 S type
1992 Jaguar Xjs 4.0 Coupe
2003 Jaguar Xjr

Clearly the communication/interface U-code will be tricky to pin down, but I guess you could start by replacing the torque converter lock-up clutch solenoid to see if this addresses the P-code. The torque converter LUC may have a mechanical or hydraulic issue which prevents it engaging, which would necessitate its replacement/remanufacture, but the EDS6 solenoid can be swapped out with the transmission in-situ so would be a relatively straight-forward thing to try initially.