Well I had more of a fiddle with the lock. B***ger me it's fixed!
Thought I would do a how-to to help others. Sorry I'm an Engineer, not a Tradesman, so I worked it out by trial and error.
The photo shows the lock retention screw (small torx head). To access, remove the round plug in the side of the door, near the lock.
Unscrew the torx until the lock drops out in your hand.
When I played around on the workbench, I inserted the key and tried turning the lock. I found it turned by a gnat's amount when fully inserted, but obviously the key worked in the lock, or it would not turn at all. The lug that rotates with the lock barrel was impacting on the cage, stopping it rotating. See the photos.
Seems the plastic piece behind the lug (keeps it tight?) had expanded over time, pushing the lug against the cage (will be obvious when lock is out of the door). Also small crack in the plastic part.
I filed a very small amount of metal from the cage to prevent the interference, and used silicone spray to help clean off the metal filings.
Reinstalled by putting back in place and tightening the torx screw, replace the plug. Works perfectly now with both keys.
Job done and glass of wine in hand (later on).