A separator comprises a gully chamber 2 having inlet and outlet ducts 10, 12. Inlet and outlet housings 20, 22 are disposed within the separator and are connected by a bypass duct 24 which extends around the wall of the chamber 2. Flow enters the chamber through an inlet which is oriented to create a circulating flow. Flow exits from the chamber through an outlet opening provided in the outlet housing 22. The outlet opening is oriented in the direction of the circulating flow so that liquid has to reverse from that flow to pass through the outlet opening. The inlet and outlet housings 20, 22 are made from identical mouldings in which appropriate openings are formed. The bypass duct can be cut to length to position the inlet and outlet housings 20, 22 opposite existing inlet and outlet ducts 10, 12.