The file was backed up on a separate hard drive as well, but that doesn't help when you're working on a file on a pc and lose eight hours of work; I had to manually move the comments notes from the back up file to the working file, which is what took up several hours. This was necessary to avoid losing the work done of the working file while repasting the comments from the back-up.
As for having autosave on, it was but that didn't help either since that autosaved version was corrupted at the same time (by 'corrupted' I mean it could be opened but the comments had vanished).
I realise there isn't much to go on, but it did happen and really should not have done and means I for one cannot rely on SoftMaker when using comments - it was pretty disastrous and would be again if it happened again, which I wouldn't now want to risk, and since Microsoft Word has never done this to me, or for that matter to the same file, I have to used Microsoft for such files. That said, I remember Lotus Smartsuite used to sometimes do this sort of thing, but that at least has the excuse of now being legacy software!
It may just be that this arose because I'm an unusually heavy user of comments, which won't impact 99% of users, and I do appreciate without being able to replicate the problem it limits what kind of response you can give. I take it from the answer there is no easy or obvious answer to this problem I had, which is what I wanted to know, so thanks for replying.