By the way i got it to add a custom message to the login screen by using the below in function but it shows the message in the login and register screen, where as i only want it showing in the login screen.
function custom_login_message() {
$message = "Already joined us? Then enter your login details below, otherwise click here to register.";
return $message;
}
add_filter('login_message', 'custom_login_message');