Final update on this - from me, at least.
Have just purchased a multi card reader from Maplins. It was recognised without any driver installation and reacted instantaneously to a card being inserted ... sooooo .. decided to try the 'problem child' card. Recognised for correct size and the mp3 files were seen OK. Used Power Desk to attempt a reformat. Specified it as FAT and it completed in little more than a second. Put it into my main camera and it was instantly recognised. Just to further check, I switched it to Setup and Format and OK'd that. ... Perfect !!
So the card is universally usable again AND I STILL DON'T KNOW WTHIH!!!
Exit left mumbling and grinning, idiot-like!
