@Jeff @Hello71 I emailed Google and was pointed at this announcement: groups.google.com/group/recaptcha-announce/browse_thread/thr‌​ead/…

Agreed - Very nicely done! Thanks for your help and the workaround.

Further to the above, the issue seems to be cause by HTTPS Everywhere. This also occurs on SO (and I assume other SE sites). I've had the addon installed for a considerable length of time (months) and haven't had issues before - not sure if it's recently been updated or if something about recaptcha has changed - In any case, my issue is diagnosed and I'll do without the addon for now. (I can accept your answer if you'd like this closed?)

Confirmed: In safe mode, captcha shows fine. I'm disabling add-ons one by one to see what triggers it. Will be back to you shortly. This is odd as I use SO frequently and never have issues with this...

Hi Jeff, I'll try it now and get back to you.