OK I have been checking again after I did a system restart.
When I opened up my fixed tab for the forums I got the logon screen as I expected, at the bottom of that screen is the small Recaptcha icon.
However I didn't require any further interrogation I was able to click the Continue button as my firefox password manager had filled the fields. So it isn't the forum anti-spam measures that hitting you but the integration of the Google Recaptcha and the forum logon URL, I think. Why is the $64,000 dollar question.