A valve assembly comprising: a valve body (102, 196, 203) having an axial flow passage (106, 172) therethrough and a lateral bore (112, 174, 190, 202) transverse to the axial flow passage (106, 172); a gate (104, 168, 216), the gate (104, 168, 216) comprising a cylinder rotated about a gate axis, the gate (104, 168, 216) being located within the lateral bore (112, 174, 190, 202) and moveable in a longitudinal direction along the gate axis from an open position to a closed position, the gate (104, 168, 216) permitting flow through the flow passage (106, 172) in the open position and a solid portion of the outer diameter of the gate (104, 168, 216) obstructing the flow passage(106, 172) in the closed position; the valve assembly being characterised by: a slot (134, 188, 204), the slot (134, 188, 204) having a helical portion (136, 190, 206) as it extends in the direction of the gate axis; and a key (142, 184, 216) configured to travel in the slot (134, 188, 204) as the gate (104, 168, 216) moves axially; wherein: the slot is on an outer diameter of the gate and the key protrudes inwardly from the inner diameter of lateral bore (112. 174); or the slot (188, 204) is on an inner diameter of the lateral bore (190, 202) and the key (184, 216) protrudes from the gate (168, 216); the slot (134, 188, 204) and key (142, 184, 216) being configured such that when the gate (104, 168, 216) moves along the gate axis, the key (142, 184, 216) interacting with the helical portion (136, 190, 206) causes the gate (104, 168, 216) to rotate about the gate axis while moving axially from the open position to a closed position. a valve body (102, 196, 203) having an axial flow passage (106, 172) therethrough and a lateral bore (112, 174, 190, 202) transverse to the axial flow passage (106, 172); a gate (104, 168, 216), the gate (104, 168, 216) comprising a cylinder rotated about a gate axis, the gate (104, 168, 216) being located within the lateral bore (112, 174, 190, 202) and moveable in a longitudinal direction along the gate axis from an open position to a closed position, the gate (104, 168, 216) permitting flow through the flow passage (106, 172) in the open position and a solid portion of the outer diameter of the gate (104, 168, 216) obstructing the flow passage(106, 172) in the closed position; the valve assembly being characterised by: a slot (134, 188, 204), the slot (134, 188, 204) having a helical portion (136, 190, 206) as it extends in the direction of the gate axis; and a key (142, 184, 216) configured to travel in the slot (134, 188, 204) as the gate (104, 168, 216) moves axially; a slot (134, 188, 204), the slot (134, 188, 204) having a helical portion (136, 190, 206) as it extends in the direction of the gate axis; and a key (142, 184, 216) configured to travel in the slot (134, 188, 204) as the gate (104, 168, 216) moves axially; wherein: the slot is on an outer diameter of the gate and the key protrudes inwardly from the inner diameter of lateral bore (112. 174); or the slot (188, 204) is on an inner diameter of the lateral bore (190, 202) and the key (184, 216) protrudes from the gate (168, 216); the slot is on an outer diameter of the gate and the key protrudes inwardly from the inner diameter of lateral bore (112. 174); or the slot (188, 204) is on an inner diameter of the lateral bore (190, 202) and the key (184, 216) protrudes from the gate (168, 216); the slot (134, 188, 204) and key (142, 184, 216) being configured such that when the gate (104, 168, 216) moves along the gate axis, the key (142, 184, 216) interacting with the helical portion (136, 190, 206) causes the gate (104, 168, 216) to rotate about the gate axis while moving axially from the open position to a closed position.