Abstract: A non-return device (10) comprises a flat silicone rubber tubing (50) defining a pair of walls (70, 90) which separate to admit fluid in a forward direction but remain together to block fluid flow in a reverse direction. A tubular body (30) surrounds the tubing (50) and comprises an off-set outlet (34) to provide an uninterrupted fluid flow surface for use in a non-vertical orientation. The tubing (50) is mounted on a cartridge (130) which moves axially within the tubular body and compresses the tubing against the tubular body to effect a seal when the device is mounted on an outlet. The tubing (50) provides good operational characteristics and sensitivity as a consequence of having a wall thickness which is 0.3% to 3% of its width.