sussed it.
It had had a Kph to Mph converter chip fitted behind the instrument pack, incorrectly..
A few question to the owner - it only started doing it after the chip was fitted.
The gearbox was waiting for the vehicle to hit (say) 65Kph (Rpm red line in whatever gear), and it was doing 65Kph, but the speedo was reading 40 so it did not change gear.
Correct the wiring to the converter chip & instruments, and all is now good