A locking mechanism comprising: a piston element comprising a piston groove extending in an axial direction along a radially outer surface of the piston element; a housing comprising a bore for receiving the piston element for reciprocating axial movement between a locked position and an unlocked position; a locking ball received in the piston groove; a locking ring mounted for rotational movement about the piston element between a locking position and an unlocking position, the locking ring comprising a radially inner locking ring groove, the locking ring groove having a recess formed in a radially inner surface thereof for receiving the locking ball in the unlocking position of the locking ring; the piston groove comprising a first axially extending section and a second axially extending section, the radial depth of the first axially extending section being greater than the radial depth of the second axially extending section; wherein the mechanism is configured such that when the piston element is in its locked position and the locking ring is in its locking position, the radially inner surface of the locking ring groove is in alignment with the piston groove such that the locking ball received in the first axially extending section of the piston groove is prevented from moving outwardly in a radial direction an amount sufficient to enter the second axially extending section of the piston groove; and when the piston element is in its locked position but the locking ring is in its unlocking position, the recess in the radially inner surface of the locking ring groove is in radial alignment with the piston groove such that the locking ball received in the first axially extending section of the piston groove can move radially outwardly into the recess, thereby allowing the locking ball to enter the second axially extending section of the piston groove and an axial movement of the piston element from its locked to its unlocked position under a force biasing the piston element towards the housing. a piston element comprising a piston groove extending in an axial direction along a radially outer surface of the piston element; a housing comprising a bore for receiving the piston element for reciprocating axial movement between a locked position and an unlocked position; a locking ball received in the piston groove; a locking ring mounted for rotational movement about the piston element between a locking position and an unlocking position, the locking ring comprising a radially inner locking ring groove, the locking ring groove having a recess formed in a radially inner surface thereof for receiving the locking ball in the unlocking position of the locking ring; the piston groove comprising a first axially extending section and a second axially extending section, the radial depth of the first axially extending section being greater than the radial depth of the second axially extending section; wherein the mechanism is configured such that when the piston element is in its locked position and the locking ring is in its locking position, the radially inner surface of the locking ring groove is in alignment with the piston groove such that the locking ball received in the first axially extending section of the piston groove is prevented from moving outwardly in a radial direction an amount sufficient to enter the second axially extending section of the piston groove; and when the piston element is in its locked position but the locking ring is in its unlocking position, the recess in the radially inner surface of the locking ring groove is in radial alignment with the piston groove such that the locking ball received in the first axially extending section of the piston groove can move radially outwardly into the recess, thereby allowing the locking ball to enter the second axially extending section of the piston groove and an axial movement of the piston element from its locked to its unlocked position under a force biasing the piston element towards the housing.