Thanks to Laidback for getting me back on the road.
The tyre was paired with a correctly sized tube on Friday and by Saturday morning it had come off the rim again - whilst sitting all by itself !?! Think this proves it is a knackered tyre.
A re-tyred (studded) wheel was waiting for me. I'm keeping studs on for a while yet. See "There's a higher chance of getting snow over Easter in the UK than there is at Christmas."