Hello Alan,
I had a very similar issue on my CAV with the ZF Transaxle.
The clutch pedal would get rock hard after the car warmed.
I could not drive it due to the lack of clutch action.
Please keep in mind that my car was four years old and not recently
constructed as yours is.
I called Ian at CAV Canada and he was spot in with his diagnosis.
He said that the ZF output may not be centered into the
pilot bearing/bushing on the back of the crank.
After replacing the external slave, bleeding, and much frustration
I removed the ZF unit and found that the throwout bearing
was almost frozen on the output shaft of the ZF unit and the
throwout was burned a blue steel color. Ian was correct.
I replaced the output shaft and the throwout and then machined the bellhousing so it was flat on both sides centered the bellhousing on the engine with a dial indicator and offset dowel pins.
After re-assembly with a new clutch disc and bronze pilot bushing all has been operating well.
I am not saying that this is your issue and I hope it is not your issue but it certainly was mine.
Please keep me informed because I hope that ypur clutch issue
has a much easier cause and solution.
I can E. Mail pics of the work and parts if you are interested.
Eric