Thanks @swarfendor437 , I will take a close look at that. I've got to say though that the new Office libre (August '24) says it is fully compatible with Office 365. I've been too busy to check with version I'm running on 17.1 Core. As you know the only issue was Excel spreadsheets... I tried Powerpoint and they worked really well, even the animations were perfect. The only issue was the 
showed as a weird symbol but I guess that means installing a compatible font.
I don't know when I'll get time to check things out thoroughly because I only got my timetable today! There are 3 courses I've not taught before, so I've got my work cut out to be classroom-ready by Monday! We don't even have a "half term" holiday here, it's straight through to Christmas with one or two long weekends, which are usually spent grading exams and writing reports!
My end goal is to be Linux dependent when support for Windows 10 ends, and be in a position to demonstrate a viable alternative to my colleagues before then.
I'll read up on Calligra Office while I'm eating my dinner. Thanks as always for your suggestions and support.