Update - I just found Removable Media under System Settings. *But* . . .
everything there (including "other media") is set to "ask what to do".

A few minutes older but none the wiser

Try setting an action you don't want as default then change back to
'ask me every time'. Works when Windows gets a flag messed up so worth
trying in Ubuntu

good idea but no joy unfortunately - even
after a restart.

I'll be reinstalling sooner or later, maybe that'll fix it

Another thought - have you a USB media player that you've used on this
system ? That could have be set to auto-run media player and the USB
card reader may be considered the same sort of device. Not sure how
you'd got over that if that's the case - perhaps plug the USB player
in and change the default application for it to 'ask'. Note that the
option won't be there until the player is present and mounted.