So fingers and whiskers crossed for my new card. I will probably report what happened, so that people who will have the same problem in future can add it to their information gathering exercise.
Ah, may the power of your whiskers triumph!
I wish these cards were clamped more tightly, as they are in phones; this one is going nowhere, and I'd glue it in if I thought it would help. I was thinking of getting one for the new 640XL, but maybe I'll check on that forum, see if the phones have a happier experience of these more generous storage solutions.
As for the Surface, I did learn that getting Device Manager to check for hardware changes can rediscover the card, which is at least quicker than rebooting or re-inserting (way too fiddly).
Still, only a few days (probably) until Windows 10 kicks off for Insiders, so I'll be monitoring system behaviour in general for a wee while. Eyes, teeth and ears crossed.
Best of luck,
Rob