Is something wrong with the e-mail notifications system?
I've had several PMs today, and I've not seen any e-mail notifications.
Nope. No errors flagged up nor bounced emails from incorrect profile email address. Check your ISP spam box?
I've not received any for months - seriously. Then, all of a sudden, I had a few the other day. And they've now stopped again. 
I've changed nothing.
You've got nothing to ein, zwei, drei, vier
Please check your ISP (in these cases ntlworld and blueyonder) webmail accounts and check the spam box. If there are notifications lurking there then whitelist the email address to stop your ISP being 'helpful'.
They were in the spam box. Why they suddenly started going in there, I have no idea.