An insert is provided for insertion through an opening in the surface of a conduit, the insert comprising an insert body having first and second open ends defining an axis of flow therebetween. The insert further comprises a valve flap connected to the insert body. The valve flap is moveable between a closed position in which it creates a closure, thereby preventing flow along said axis, and an open position in which the valve flap allows flow along the axis. The valve flap is arranged so that, when in the open position, it is substantially U-shaped in side cross-section and is oriented with respect to the insert body so that said U-shape arches over the axis of flow.