Trying to change login image, please can someone advise?

Hi

Was just following another thread where I saw advice to add the following to my functions.php file;

// custom admin login logo
function custom_login_logo() {
	echo '<style type="text/css">
	h1 a { background-image: url('.get_bloginfo('template_directory').'/images/custom-login-logo.png) !important; }
	</style>';
}
add_action('login_head', 'custom_login_logo');

However, having done this, and having added the full url of the destination of the image I want to use, all I get now is no image at all above the login boxes.

Please can someone tell me what I’ve done wrong? I’ve tried both including the http part and just starting at the / after the .co.uk, but nothing works :/

Thanks