Hello Huw66.
Thank you for using Apple Support Communities and for sharing these details. Sounds like you're unable to successfully open specific spreadsheets in the Numbers app. When you attempt to, they just load/spin and nothing happens. We can understand how this would be an issue. Allow us to investigate this issue and assist in any way we can.
Ever since this issue started to occur, have you been able to restart your iPad Air to see if that helps with this specific issue?
Restart your iPhone - Apple Support
Additionally, if you have any pending updates available for your iPad Air, it would be best to install them. Keeping your device up-to-date, can help resolve various issues and help with overall performance. Just in case, the resources below can guide you through the update process:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
We hope this information is useful. Cheers.