Progressively getting worse, my key gets stuck in the ignition barrel when switching the engine off - it does not want to rotate that final click before removal. It now takes a fair bit of frantic waggling before it releases, though sometimes it surprises me and doesn't happen (a rare occurrence now)

Has anybody else had this issue, and can it be cured with a deft application of Servisol (spray isopropanol) or an aerosol silicone lubricant ?

 

sounds like you are shutting down with the steering wheel in a slight bind. Have you tried tweaking the wheel a bit while trying to remove the key?

 

That was on the suspect list, but doesn't appear to be the culprit. Although I will try changing the steering wheel reach to see if it makes a difference. I think something in the barrel is the most likely suspect.