Hello, @JonFarmer69, the information you have provided is not enough for a complete understanding of the situation.
The first question is about your Outlook version number and build, please give more info. To see it, go to File and then Account Settings.
In the folder list, click the keyboard combination Ctrl+6 and see if you have the section of all group calendar. If the answer is yes, you should see the All Group Calendars group on the calendar module it is usually the last group and it can maybe not visible if you have a big amount of calendars. (Ctrl+2)
If you don't see the all calendar group in the calendars, but it is in the Folder list, my recommendation is to restart the Outlook app with the help of /resetnavpane
Now you can close Outlook and right-click the Start button. Type this in it: outlook.exe /resetnavpane
Confirm it by hitting Enter, and restarting Outlook. Check this helpful thread for more details.
One more recommendation from me is to be always prepared for the recovery of your Microfot 365 account, as in our times, data is your biggest asset.
