Miele G646 C Plus Pump running continuously?

I have cleaned out the pump, filters, non return valve and checked the drain hose is clear. It seems that the sensor that detects the water level may be faulty as if you rock the machine gently (disturbing the water inside) the pump stops.

If you put some water inside - disconnect the drain hose and shut the door, the pump will run and the water flows freely out of the drain hose, but the pump does not stop. When you look inside, there is only a small amount of water left in the bottom of the sump, so the pump does seem to be working OK.

Any ideas anybody?