Thanks to all who have posted on this. Both my keys have needed jiggling for a while...then yesterday neither would turn. Eventually one did and I got home, resolving to sort things today. Undid the 2 little crosshead screws and took off the ignition switch so I could turn it with a screwdriver...but checked the barrel. It turned perfectly. Put it all back and it works fine.
Just wish I knew why

Any ideas? Apart from carry the tools in case it acts up again