A Microsoft file hosting and synchronization service.
Hi, after about a week of this and going through several Microsoft engineers, escalating the issue up the chain of expertise, I still hadn't solved this. I tried everything, even setting up an account on an old Windows 10 computer, no luck. I had a rest from it today and went and did some gardening, and the answer just popped into my head! I had realised that any document saved to Desktop, whether new or old always worked properly, whereas all those in the documents folder did not. Any Word files saved elsewhere (such as in among the photo files) also worked. So, the logical conclusion was that there was a problem with the file path to Documents. Perhaps I accidentally moved something, I don't know, the file path had "Documents - Documents" in it, which I reckon was one too many Documents. Anyway I deleted the Documents folder and all its contents from my computer and OneDrive and then re-added them from my portable hard drive into the OneDrive Documents folder on my computer. Everything uploaded to OneDrive again and all the documents are working as they ought to. Autosave is also On, except for Compatablity Files but that's easy to rectify. No sign of d.docs live or files being renamed to (number) and so on.
I am so relieved, and hopefully someone will find this helpful in the future.