[ShaneHobson]

Gave up and called an automotive locksmith.
He removed the interlock solenoid off the ignition switch, removed the key barrel, cleaned, lubed, reassembled (minus the interlock solenoid) and it all works perfectly now.

He thinks either the interlock solenoid was faulty or I had turned the key when it wasn't fully inserted.

Solenoid is now detached from ignition, but still connected to the loom. Tied up out of the way now.

The locksmith made a new key which is what I use daily now, with the warn key as a spare.

Thank you to everyone who offered suggestions.