Thanks to the last few posts (cheers Dave) I've had a lighbulb moment and fixed recaptcha with FEC

Because FEC doesn't use the default zencart register page - it uses the login page as a split register / login page the observer wasn't looking for the recaptcha on that page

In includes/classes/observers/class.google_recaptcha.php

Add $pages_to_check[] = 'NOTIFY_LOG_IN_CAPTCHA_CHECK'; to the array of pages to check so it becomes:

Code:
	function __construct() {

		//comment out any pages you do NOT want to check
		$pages_to_check[] =  'NOTIFY_CONTACT_US_CAPTCHA_CHECK';
		$pages_to_check[] =  'NOTIFY_CREATE_ACCOUNT_CAPTCHA_CHECK';
		$pages_to_check[] =  'NOTIFY_REVIEWS_WRITE_CAPTCHA_CHECK';
		$pages_to_check[] =  'NOTIFY_LOG_IN_CAPTCHA_CHECK';
		$this->attach($this,$pages_to_check);
	}
Then in the same file update the following

Code:
if (!$resp->isSuccess()) {
			$event_array = array('NOTIFY_CONTACT_US_CAPTCHA_CHECK' => 'contact', 'NOTIFY_CREATE_ACCOUNT_CAPTCHA_CHECK' => 'create_account', 'NOTIFY_REVIEWS_WRITE_CAPTCHA_CHECK' => 'review_text', 'NOTIFY_LOG_IN_CAPTCHA_CHECK' => 'create_account');
			$messageStack->add($event_array[$eventID], $resp->getErrors());
			$error = true;
		}
This now seems to be working fine - although there is no error is you don't tick the recaptcha? (must have missed something)