Ah… worked it out, though normally I rely on the traffic of the blog to hit wordpress’ fake cron I did have a daily crontab with wget to make sure site was fresh just before doing an export for an integration… but to stop it downloading the response of the request (in this case an empty file) you need to add the –spider option
31 0 * * * wget -qT 30 --spider http://www.example.com/wp-cron.php