is_page within a plugin

Hi, diving into updating a plugin – Members only – so that it only requires login for a pages. The original code looks like this

function members_only()
{
	global $userdata, $members_only_opt, $members_only_reqpage;

	$home = get_bloginfo('url'); //Get base URL or WordPress install
	$currenturl = sprintf('http%s://%s%s',(isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == TRUE ? 's': ''), $_SERVER['HTTP_HOST'], $members_only_reqpage); //Get the current URL

	//Check redirection settings
	if ($members_only_opt['redirect_to'] == 'login' || $members_only_opt['redirect_to'] == 'specifypage' && $members_only_opt['redirect_url'] == '') //If redirecting to login page or specified page is blank
	{
		$redirect = "/wp-login.php";
		if ($members_only_opt['redirect'] == TRUE) //If redirecting to original page after logging in
		{
			$redirect .= "?redirect_to=";
			$redirect .= $members_only_reqpage;
		}
	}
	elseif ($members_only_opt['redirect_to'] == 'specifypage' && $members_only_opt['redirect_url'] != '') //If redirecting to specific page
	{
		$redirect = '/'.$members_only_opt['redirect_url'];
	}

	//Create Redirection URL
	$redirection = $home.$redirect;

	if ($userdata->ID == '' && $members_only_opt['members_only'] == TRUE)//Check if user is logged in and blog is Members Only
	{
		//Check we aren't already at the page we're redirecting to (with and without the trailing slash) or wanting to go to the login page, registration page or somewhere in wp-admin
		if ($currenturl == $redirection || $currenturl == $redirection.'/' || preg_match("/wp-login.php/i", $_SERVER["REQUEST_URI"]) || preg_match("/wp-register.php/i", $_SERVER["REQUEST_URI"]) || preg_match("/wp-admin/i", $_SERVER["REQUEST_URI"]))
		{
			//Do Nothing
		}
		else
		{
			//Redirect Page
			ob_start();
			header("Location:".$redirection);
			ob_end_flush();
		}

	}
	else
	{
		//Do Nothing
	}

}

All I am doing is putting in a test to see if it is a page…

{
	if (is_page())
	{
		//Redirect Page
		ob_start();
		header("Location:".$redirection);
		ob_end_flush();
	}
        else
        {
                // Do nothing
        }
}

The is_page() function just always appears to return false.

any pointers.

thanks