A fluid flow control valve comprises a body (11) in which is a valve chamber (14) connecting an inlet to an outlet (15) for a source of fluid, with a flexible diaphragm (18) opening or closing the outlet, in use. An open end of the body is closed by a cover (19), and received in the interior of the cover is a cap-like member (32), between which member (32) and the diaphragm (18) is defined a sub-chamber. The sub-chamber communicates with the source of fluid by way of a bleed hole (18a) in the diaphragm (18) and the outlet from the sub-chamber is a passageway through an outlet tube (32) on a flexible front wall (34) of the member (32), this tube extending through an opening in the cover (19) to contact a sealing pad (31) on an arm (29) of a float pivotally mounted on the cover. The position of the outlet from the sub-chamber is thus movable, due to the flexibility of the front wall (34), and this allows water hammer to be reduced/eliminated.