miele premier 300 washing machine?

when the washing cycle has finished, the drain pump runs for a very long time, could this be due to a faulty water level sensor? If so how do i get to it to test it, where is it in the macine?, how do i get the side off the machine to access the other end of the sensor pipe?
I have cleaned all the outlet pipework from the back of the machine to the sink drain, also the pump filter, so i don't think it is a drain problem