Cron – “NTP socket is in use” messages from ntpdate in syslog

In cron.log I can see that ntpdate is being triggered from a cron job which is running daily at 23:15

Mar 10 23:15:01 seitan /USR/SBIN/CRON[22407]: (root) CMD (/etc/network/if-up.d/ntpdate)

But I cannot seem to find the cron job that is causing these messages to fill up my syslog

Mar 10 23:15:01 seitan ntpdate[22416]: the NTP socket is in use, exiting

I'm guessing I need to change this cron job to stop the ntp service before this command executes, then start it again afterwards, but I don't know where this is being triggered from. I have checked the cron.daily directory and it only contains a file ntp which only seems to rotate the ntp stats (contents below):

#!/bin/sh

# The default Debian ntp.conf enables logging of various statistics to
# the /var/log/ntpstats directory.  The daemon automatically changes
# to a new datestamped set of files at midnight, so all we need to do
# is delete old ones, and compress the ones we're keeping so disk
# usage is controlled.

statsdir=$(cat /etc/ntp.conf | grep -v '^#' | sed -n 's/statsdir \([^ ][^ ]*\)/\1/p')

if [ -n "$statsdir" ] && [ -d "$statsdir" ]; then
    # only keep a week's depth of these
    find "$statsdir" -type f -mtime +7 -exec rm {} \;

    # compress whatever is left to save space
    cd "$statsdir"
    ls *stats.???????? > /dev/null 2>&1
    if [ $? -eq 0 ]; then
            # Note that gzip won't compress the file names that
            # are hard links to the live/current files, so this
            # compresses yesterday and previous, leaving the live
            # log alone.  We supress the warnings gzip issues
            # about not compressing the linked file.
            gzip --best --quiet *stats.????????
            return=$?
            case $return in
                2)
                    exit 0                  # squash all warnings
                    ;;
                *)
                    exit $return            # but let real errors through
                    ;;
            esac
    fi
fi

I have never touched any of the ntp configuration in the system.