The present invention relates to a position adjustment mechanism, comprising two cylindrical portions, a first one of the portions being slidably disposed inside a second one of the portions. One of the portions has at least three detents and the other portion has at least three members for engaging in respective the detents to hold the portions in a first position, the members being removable from the detents to allow the portions to move into a second position; wherein the detents and members are equi-spaced around the first and second portions.