And because it's a Google account I can't add it to the list of alternate emails for the Google account I _can_ use with Google+.
So Google+ emails can only go to the email address associated with this account - which is one I didn't have until this evening, and which I have no particular wish to ever receive email at, ever!
Ok - what's the best way to forward emails from one GMail account to another? All I want is Google+ notifications (which is all that should ever show up there) to appear in my @ducker.org.uk account rather than this new @gmail.com account.
Edit: Got it. GMail actually handles forwarding really well. Checked for confirmation, set it up with some nice options, and it all works perfectly. The only glitch I had was that I wasn't able to test it myself - as emails from firstname.lastname@example.org sent to an account that autoforwards _to_ email@example.com don't actually get forwarded, because that might create an infinite loop! So, smart on their behalf, momentarily baffling for me. All sorted now though.
Original post on Dreamwidth - there are comments there.