Im trying to display the error page in /temp/www/error403.html whenever a 403 error occurs.
This should be whenever a user tries to access the site via https (ssl) a
It looks like there's a boo-boo in the listed configuration, as it is only sending error code 503 ("service unavailable") to the custom page, so for 403 ("forbidden") you probably want to use:
error_page 403 /error403.html