Overflowing machine but pressure switch okay?

My washing machine continues filling, and then overflows without the timer moving on. Sounds like the pressure switch. I have disassembled the pressure switch and it is clean as a whistle. The internals are simple mechanical switches that appear to work well; nothing much to go wrong in fact. The pipe that goes between the pressure switch and the filter looks fine and I can blow through it easily; it is not split. I notice that there is another, similar thin black pipe that goes from the filter to somewhere at the front of the drum, perhaps to the door catch (I can't see). What does this pipe do? and could it cause the problem? If I have to remove the filter how can I easily do it on a zanussi FL828 (the front and side of the machine wrap around making it difficult to get to)? Thanks for your help.