A valve (10, 100, 400) comprising: a valve body (12, 120) including an inclined valve seat (14, 140) defining a fluid aperture (15, 150); a valve head (20, 200) for contact with the valve seat for closure of the fluid aperture; a lever arm (22, 220) attached to the valve head; and biasing means (32, 320) operatively coupled to the lever arm for urging the valve head into contact with the valve seat, said biasing means being arranged to provide decreased biasing on opening of the valve head relative to the fluid aperture, wherein the valve seat (14, 140) is inclined at an angle of from 5° to 80° relative to a transverse plane through the valve body (12, 120); the lever arm comprises a pair of elongate lever arms (22, 220) defining a longitudinal axis offset from the valve head, each of the lever arms having a first end and an opposite second end, the first end of each of the lever arms being connected to the valve head (20, 200), the lever arms being rotatable about a pivot axis located between their first and second ends, the lever arms being laterally spaced apart and defining an elongate space therebetween; the biasing means comprises an elongate tension spring (32, 320) having one end anchored to a first anchor point located between the lever arms at the second end and an opposite end anchored to a second anchor point on the valve body; an angle made between a longitudinal axis of the spring and a line drawn between the pivot axis and the first anchor point decreases from a value when the valve head is closed to substantially 0° when the valve head is fully open; and a portion of the spring enters the elongate space between the lever arms as the valve head opens and exits the elongate space as the valve head closes. a valve body (12, 120) including an inclined valve seat (14, 140) defining a fluid aperture (15, 150); a valve head (20, 200) for contact with the valve seat for closure of the fluid aperture; a lever arm (22, 220) attached to the valve head; and biasing means (32, 320) operatively coupled to the lever arm for urging the valve head into contact with the valve seat, said biasing means being arranged to provide decreased biasing on opening of the valve head relative to the fluid aperture, wherein the valve seat (14, 140) is inclined at an angle of from 5° to 80° relative to a transverse plane through the valve body (12, 120); the lever arm comprises a pair of elongate lever arms (22, 220) defining a longitudinal axis offset from the valve head, each of the lever arms having a first end and an opposite second end, the first end of each of the lever arms being connected to the valve head (20, 200), the lever arms being rotatable about a pivot axis located between their first and second ends, the lever arms being laterally spaced apart and defining an elongate space therebetween; the biasing means comprises an elongate tension spring (32, 320) having one end anchored to a first anchor point located between the lever arms at the second end and an opposite end anchored to a second anchor point on the valve body; an angle made between a longitudinal axis of the spring and a line drawn between the pivot axis and the first anchor point decreases from a value when the valve head is closed to substantially 0° when the valve head is fully open; and a portion of the spring enters the elongate space between the lever arms as the valve head opens and exits the elongate space as the valve head closes.