Well, I reverted to 1.15 (DP24)... and hit a 'file error' (plus one other similar message) when I was auto-punching several takes.
However, I don't think I'd remembered to reformat the card after downgrading the firmware from 1.18. Did so and it's been fine since (with lots of auto-punching takes).
I'm using a card that I've used for the last couple of years (entirely without problems until lately):
SanDisk 32GB SDHC, marked 'I', '4'.
I guess that the 1.18 (or equivalent updates for the other machines in the range) might just be choosier about cards? If so, will be good to find out what to use.