Long story short...

The problems were caused by a whole bunch of templates being missing from the system database.

Don't quite understand how this happened, nor when they disappeared but they appeared to be missing from the database back in April 2017 when I transferred the site to my server, so I have to say again... have no idea how this has happened but I suspect it was during the upgrade to 4.2.4 which was, if memory serves correctly, completed when the site was on the GoDaddy platform. I recall having ALOT of problems with the upgrade script timing out and needing to be re-run so my guess is it didn't complete properly and something went horribly wrong.

The only fix I was able to implement was completing an upgrade to the latest version which appears to have fixed it.

So, if you notice any problems please let me know immediately.

Apologies for the inconvenience.