John Spackman, an ace admin on a related forum, kindly agreed to look into this issue about 10 days ago. Since then I've been in Italy and then straight onto Jury Service, so I haven't been around much; but somewhat ominously, nothing from John yet (and he's usually very quick with these issues).
I can confirm there is a problem, but was personally (with my limited admin skills) unable to see any cause. All looked OK from what I could see in the forum admin screens, just no emails being sent out on new posts to subscribed forums (or PM notifications), so very frustrating.
Thanks for the reminder, I'll send John an email now and ask if he's made any progress.
EDIT: email now sent to John, so I hope he has some news....