OK - over the weekend, the time did slip, however I have found another bit of info which seems to set the ntp.conf file and fix it.
NTP Clients
Again the following should be done in the described order.
On Mac OS X Clients and on those Mac OS X Servers that are to act as NTP clients (that means they query a time server but do not answer NTP queries) the following needs to be done:
In System Preferences ("Date and Time" pref pane) "synchronize time automatically" must be ticked; then the first tab of this pref pane must be activated and the window left open. Then in Terminal a time server to sync with must be added to /etc/ntp.conf like so (as to finding the right time server see above):
server mein.zeitserver.de prefer iburst minpoll 12 maxpoll 15
Now /etc/ntp.conf must be saved and made unchangeble:
sudo chflags uchg /etc/ntp.conf
After that the System Preferences can be closed.
On a Mac OS X Client that is all. On a Mac OS X Server the file /etc/hostconfig must be edited to contain:
TIMESYNC=-YES-
Still working on it