Any Corsa owners on here ? Or mechanics !

[Ninja]

Ivor, maybe ring a garage and explain that you suspect you have a sensor failure and ask for a rough repair quote.
I'm no qualified mechanic but I do know a bit about cars and that is what it sounds like.
It might be a small and cheap job although I wouldn't know for sure. I've had a few sensors go on cars and usually just leave them if not needed

[Bluebeard]

Hi Ivor. I had an Astra many moons ago with a Board Computer (BC) that gave similar readouts. It could either give average figures or instantaneous numbers. It sounds like yours is set to give instantaneous data so it will show 2 if you accelerate hard or 999 if you are engine braking.

If you can see a BC button try pressing it to see if the readout changes to one that annoys you less!