The present invention provides a non-return valve for a resin injection system, especially for use in electrical cable joints, comprising: a valve housing which includes an outlet for a fluid flowing through the valve; and a valve body which is held by the valve housing and is freely movable between a closed position, in which the outlet is substantially sealed or closed to fluid flow by the valve body, and an open position, in which the outlet is substantially open to fluid flow. Thus, with the present invention, the valve body can be loosely held by the valve housing and the valve need not include any spring means to bias the valve body to the closed position. The invention also provides a resin injection system for sealing a localized breach in an object, such as an electrical cable joint.