Hi Daimler Jack,
It sounds to me like you have a governor or kickdown switch/cable problem with your transmission if it refuses to change up when bouncing off the rev-limiter. Have you tried backing off the throttle slightly so the pedal isn't nailed against the kickdown switch to see if it changes up then? What happens if you change it down and then up manually at the rev limit?
I'm afraid I don't have much expertise with the ZF4 as installed in early 40s, but if it is anything like the installation in other cars at the time I would start to look at things like kickdown cable/switch adjustment if it only happens at full throttle. If the transmission in the earlier cars gets a full throttle signal from the throttle position sensor like the later cars do it could also be a dirty TPS (I'm sure someone like Naki will correct me if the early cars don't use the TPS for transmission signals), but then I would expect to see a transmission fault light.
If it is a sticking governor you could try doing a filter and fluid change on the transmission and see if running clean fluid helps (or take it to a transmission shop and get them to flush it), but otherwise the governor will have to be disassembled and sorted (transmission shop job really). From memory the governor is towards the back of the transmission on the ZF4 and it can be worked on without removing the transmission from the car.
'94 XJ12 (and other toys)