Hello Tom. I think perhaps the 3x50mm hardened steel pin has sheered, that operates the plunger, mine did the same a few months back. It's quite an easy fix once you remove the lock assembly. Pins available on eBay, but I have some spares if you need just one.
After a very lengthy period, at long last, I stripped out the door lock mechanism. Looks as if shear pin was the culprit.
Intend to clean everything back and rebuild.