Yes other SD cards are recognised and yes I have cleaned the contacts of the affected card.
It may be an issue with the individual card but (and I am NOT a technical expert) it is weird that the sDcard is seen in the computer and is mounted without issue when inserted into the PC.
To note though that while I say that other SD cards are recognised, they first (for a second or two) show as “Memory card not inserted” before being recognised a moment later in the C2 settings.
It is odd too that the Books App, after importing two books from Download, showed two .mobi books as being on the (suspect) sDcard but did not show them when the sDcard had been swapped for another.
Perhaps it is just a dodgy SDcard (SanDisk Ultra bought new from Amazon) so I’ll use another SD for now thanks.