Abstract: A non-return valve mechanism comprises a tubular member (1) having a substantially constant external diameter whereby the tubular member can be inserted into a pipe (5) from an open end thereof. A pivotable closure member (9) is provided at one end of the tubular member for closing the end of the tubular member, the closure member being adapted to be positioned externally of the pipe. At least one seal (3) extends around the external periphery of the tubular member for sealing with an internal surface of the pipe into which the tubular member is inserted.