I do (thankfully) have a backup.
I had an unplanned visit to a client today, so took the opportunity to call into the local Apple store on the off chance and managed to speak to one of the guys on the Genius Bar. As I had the Mac and back up drive with me he was able to sort out the main problem. Still got a load of duplicate entries (hours of endless fun ahead in removing the more serious examples) but at least I got back the missing items.
Appreciate you taking the time to respond to my query.
I'm glad you have it fixed.
If you are using iCloud, I suggest you copy the calendars to a local copy before using it though, as iCloud tends to not take the deletions, let me know if you'd like help with that.
As I sell software on my site and ask for donations, the Apple Discussions Use Agreement requires that I state that I may receive some form of compensation, financial or otherwise, from my recommendation or link.