Just a final report.

I have fixed this issue, well sort of.

It appears that the vfd forgot its settings somehow. Previously I was using the on off switch on the small sub panel below the main control section and the pot for speed control. Never using the on, off or menu controls on the main panel.
When I downloaded the manual and started going through a setup process I found it wanted to run through the main screen and other buttons. This is now working as it should, probably as it always should have but when I bought it I just did what the previous owner had done.
Interestingly the other switch and pot I was using no longer have any effect.
I think it must have been set up to run externally and now it is running internally ( I forget the correct term ). I still don`t know why it decided to stop working and don`t 100% trust it but it works for now.

Ollie