Thanks byte, here is the contents of my /etc/cron.d/ directory:
-rw-r--r-- 1 root root 3 Dec 19 22:56 backup
-rw-r--r-- 1 root admin 84 Jan 14 15:55 backup2ws.476035
-rw-r--r-- 1 root root 88 Jan 15 11:33 clamav
-rw-r--r-- 1 root root 188 Feb 22 2005 sysstat
-rw-r--r-- 1 root root 111 Mar 8 2002 warnquota
Now how do I work out which process is causing the entry?