VITRA CISTERN
I've had exactly the same problem - and solved it. Turn off the water to prevent the cistern refilling.
Remove the lid off of the cistern. Flush to empty it.
Turn the large white chamber in the centre of the cistern slightly anti-clockwise and lift clear.
Mine had a white mastic seal which had come adrift and had prevented it from sealing properly.
I removed the mastic, replaced the chamber (turn clockwise to lock), turn on the water and the cistern works perfectly. I hope this works for you.
MARINER
June 2009