I've sorted it. Hooray.
I read the link and used that as my guide.
This is what I did, in case anyone needs help :-
1, copied and pasted my saved games to a memory stick
2, deleted all the original saved games
3, copied the corrupted game's backup game back to the 'saves' folder
4, renamed it, removing the backup part of the name
5, started the game up, played, & saved successfully
6, copied all saved games back to 'saves' folder
Thank you MrsFlynn for pointing me in the direction of the link I wouldn't have thought to do this without reading that.
Happy Sim'ing all