Thanks guys - I've looked at all the above.
All the days are still there - but not individually highlighted any more - I've just got one group of "Today", and one headed "Last Week" (no longer split into the last seven days) then groups of weeks.
I tried a system restore (good thinking) - but the computer refused to do it. Said it couldn't (for some reason) - even though there were valid restore points. I did notice, however, that one of those restore points for last Saturday was when Windows automatically updated itself.
I think Microsoft may have sent me some crap.