Oceanis 411 Water tank system

I have the three cabin version of the Oceanis 411 which I purchsed October 09.
When I was getting no water from neither the galley tap nor the two heads taps and the pump was running without pressurising, I decided the tanks needed filling. The one in the bow was obviously empty as it took a long time to fill, however the one in the stern was already full to capacity and overflowed as soon as I put the hose in. Can anyone help me please, as to why I am not drawing any water from the stern tank. I can't see anything in the boat manual to help me.
Thanks for reading, hope you can reply
Paul