A filter assembly has a compressible filtering medium (18, 19) disposed in the flow path of water from the upstream region of an inlet chamber (11) to a downstream outlet (17). Said compression of the filter medium by the water passing through the filter assembly increases the proportion of the overall flow path of water constituted by a low resistance portion of the flow path. The clogging of the filter medium thus leads to an increase in water pressure in the upstream region which, in turn, causes compression of the filter medium, which compression tends to increase the water flow rate through the outlet so as to counter the reduction in water flow rate causes by the clogging.