A backflow prevention valve comprising: a valve body defining a chamber; an inlet opening into the chamber and arranged on an inlet axis; an outlet opening from the chamber and arranged on an outlet axis, wherein the outlet axis is substantially collinear with the inlet axis; and a funnel arranged in the chamber around the outlet, the funnel tapering from a first larger perimeter at a first end to a second smaller perimeter at the outlet; wherein the chamber includes an upper portion extending from the inlet to a lower edge of the upper portion, the lower edge having a third perimeter which is larger than the first perimeter of the funnel, wherein the chamber has one or more backflow outlet ports arranged between the first perimeter of the funnel and the third perimeter of the upper portion; wherein the inlet has a flow path therethrough having a substantially cylindrical bore; whereby the backflow prevention valve further includes inlet flow path adjustment means adapted to control the internal diameter of the flow path through the inlet; characterised in that the distance between the inlet and the backflow outlet ports measured in the direction of the inlet axis is at least twice as large as the internal diameter of the cylindrical bore of the flow path through the inlet; and in that the inlet flow path adjustment means comprise one or more interchangeable inserts arranged to be selectively located in the inlet to control the internal diameter of a flow path through the inlet, and in that the backflow prevention valve further includes outlet flow path adjustment means comprising one or more interchangeable lower funnel portions arranged to be selectively fixed to the lower end of an upper funnel portion to control the internal diameter of a flow path through the outlet. a valve body defining a chamber; an inlet opening into the chamber and arranged on an inlet axis; an outlet opening from the chamber and arranged on an outlet axis, wherein the outlet axis is substantially collinear with the inlet axis; and a funnel arranged in the chamber around the outlet, the funnel tapering from a first larger perimeter at a first end to a second smaller perimeter at the outlet; wherein the chamber includes an upper portion extending from the inlet to a lower edge of the upper portion, the lower edge having a third perimeter which is larger than the first perimeter of the funnel, wherein the chamber has one or more backflow outlet ports arranged between the first perimeter of the funnel and the third perimeter of the upper portion; wherein the inlet has a flow path therethrough having a substantially cylindrical bore; whereby the backflow prevention valve further includes inlet flow path adjustment means adapted to control the internal diameter of the flow path through the inlet; characterised in that the distance between the inlet and the backflow outlet ports measured in the direction of the inlet axis is at least twice as large as the internal diameter of the cylindrical bore of the flow path through the inlet; and in that the inlet flow path adjustment means comprise one or more interchangeable inserts arranged to be selectively located in the inlet to control the internal diameter of a flow path through the inlet, and in that the backflow prevention valve further includes outlet flow path adjustment means comprising one or more interchangeable lower funnel portions arranged to be selectively fixed to the lower end of an upper funnel portion to control the internal diameter of a flow path through the outlet.