A system comprising: a male connector, a female connector and a release tool; the male connector intended to be inserted into the female connector, the male connector defining a conduit and comprising a locking means operable to engage the female connector to thereby lock the male connector relative to the female connector and a release means for unlocking the male connector to permit the male connector to be separated from the female connector, wherein the release means is accessible from within the conduit and comprises a sleeve, the male connector further comprising: a substantially cylindrical steel body comprising: a plurality of ball apertures which extend through the body and are circumferentially spaced around the body, each ball aperture shaped to receive a spherical locking ball, the locking balls being retained in the ball apertures on the internal side of the body of the male connector by an external side of the sleeve which always extends across the ball apertures wherein the plurality of ball apertures and the locking balls define the locking means; the sleeve comprising a plurality of substantially hemi-spherical grooves on the external side of the sleeve facing the internal side of the body of the male connector, said plurality of substantially hemi-spherical grooves being equivalent in number to the number of ball apertures and hence balls that extend through the body of the male connector, the female connector comprising a circumferential groove that extends around the internal surface of the female connector, the circumferential groove being positioned such that when the male connector is fully inserted into the female connector, the circumferential groove lies adjacent the ball apertures of the male connector, the release tool comprising a release tool body arranged to be inserted into the conduit and comprising an engagement means arranged to engage the release means to unlock the male connector relative to the female connector, the engagement means comprising one or more engagement means balls or rollers for gripping the release means. a male connector, a female connector and a release tool; the male connector intended to be inserted into the female connector, the male connector defining a conduit and comprising a locking means operable to engage the female connector to thereby lock the male connector relative to the female connector and a release means for unlocking the male connector to permit the male connector to be separated from the female connector, wherein the release means is accessible from within the conduit and comprises a sleeve, the male connector further comprising: a substantially cylindrical steel body comprising: a plurality of ball apertures which extend through the body and are circumferentially spaced around the body, each ball aperture shaped to receive a spherical locking ball, the locking balls being retained in the ball apertures on the internal side of the body of the male connector by an external side of the sleeve which always extends across the ball apertures wherein the plurality of ball apertures and the locking balls define the locking means; the sleeve comprising a plurality of substantially hemi-spherical grooves on the external side of the sleeve facing the internal side of the body of the male connector, said plurality of substantially hemi-spherical grooves being equivalent in number to the number of ball apertures and hence balls that extend through the body of the male connector, the female connector comprising a circumferential groove that extends around the internal surface of the female connector, the circumferential groove being positioned such that when the male connector is fully inserted into the female connector, the circumferential groove lies adjacent the ball apertures of the male connector, the release tool comprising a release tool body arranged to be inserted into the conduit and comprising an engagement means arranged to engage the release means to unlock the male connector relative to the female connector, the engagement means comprising one or more engagement means balls or rollers for gripping the release means. a plurality of ball apertures which extend through the body and are circumferentially spaced around the body, each ball aperture shaped to receive a spherical locking ball, the locking balls being retained in the ball apertures on the internal side of the body of the male connector by an external side of the sleeve which always extends across the ball apertures wherein the plurality of ball apertures and the locking balls define the locking means; the sleeve comprising a plurality of substantially hemi-spherical grooves on the external side of the sleeve facing the internal side of the body of the male connector, said plurality of substantially hemi-spherical grooves being equivalent in number to the number of ball apertures and hence balls that extend through the body of the male connector, the female connector comprising a circumferential groove that extends around the internal surface of the female connector, the circumferential groove being positioned such that when the male connector is fully inserted into the female connector, the circumferential groove lies adjacent the ball apertures of the male connector, the release tool comprising a release tool body arranged to be inserted into the conduit and comprising an engagement means arranged to engage the release means to unlock the male connector relative to the female connector, the engagement means comprising one or more engagement means balls or rollers for gripping the release means.