Good news, all is working again.
Formatted the card to a single 32gB FAT32 and it worked, so extended it to the full 64gB and it’s all ok. Plays everything I’ve thrown at it, just like it used to! The player reported correctly the memory card size, used and free space in both instances.
Not sure you can buy 64gB cards in anything less than UDHC class 10 format, but it works.
One thing I DIDNT do is use Windows to carry out the partitioning and formatting, I used the free and excellent MiniTool Partition Wizard - maybe that’s the issue?
Happy Days!
Paul
