'01 Mini cooper CVT gearbox problem

[Greg964]

My sisters mini has developed a gearbox problem whereby it selects too low a gear all the time so the engine is screaming when you are pottering around. It wont change up as you'd expect, or if it does change up its still too low a gear so the revs are still really high all the time. 1 time out of 5 it will fire up and drive normally, but 4 times it will have this high rev issue.
I can't find any info online (apart from people saying the CVT box is st).
She took the car to a specialist who read the ECU fault codes, but apparently there weren't any. I think the car had a replacement ECU a few years ago, but it has run fine since, until now.
Any ideas?

[DanGT]

Start by checking the oil level in the gearbox.