Hello Stephen, thanks again for your help.
I've logged into my Microsoft account and checked devices as you suggested. The iPad doesn't show and neither does my iPhone 12 Pro. The only thing showing on my account is my desktop Windows PC. The iPad is a 2017 10.5 inch iPad Pro with 64 GB - dated but still runs everything quite quickly. It has the latest iPad OS 17.4.1
I followed the steps you suggested to use if the iPad wasn't on the list. First I opened Excel, I clicked on the account icon (my initials - I presume this is correct, but it's not at the top right, it's at the top left. There's nothing related to my account at the top right) and then Account Details but I was already signed in and the only choices were to "Sign out" or to "Add Account". I tried again with Word - same result.
I then followed the instructions on my PC at www.accounts.microsoft.com to add a device. The dop down box choices were Microsoft devices, e.g. Xbox, surface laptop and at the bottom was "Other Devices". I clicked on this then used the drop down box and clicked on iOS Device. It asked me to input the serial number of my device (iPad). I did this but it said it didn't recognise it.
As previously mentioned I already have the apps, e.g. Word, Excel, OneNote, OneDrive, etc., on my iPad. I use Word and OneNote quite a lot when I'm offline so I assume I'm not using the online versions of "MS Office".
I can't get my account to see my iOS devices yet the Microsoft apps seem to work ok (apart from the external links for Excel) and I can't find a way through to add my iOS devices from my PC or from the devices themselves. I have logged into my Microsoft account both on my PC and on my iPad via both Word and Excel as per your advice. The iPad is very limited in what it will show me on my account, e.g. no devices tab, nothing other than the options to "Sign Out" or "Add account" as outlined above so I'm stuck at the moment.
I'm a great believer in "garbage in, garbage out" so I guess I must be doing something wrong. Any further advice would be welcome.
Just one last thing, I use Dropbox mostly for my files as I have an ancient MacBook Pro that won't run Microsoft 365, though I also keep copies on OneDrive as well. Could this have some bearing on the problem?
Many thanks
Stuart Gilbert