A cover for a valve (10) has a housing (20) mounting internally a component (24), with a collet (28) being located with play through an open end of the component (24). Resiliently deformable jaws (36) of the collet (28) co-operate with a profiled inner wall of the component (24) for opening and closing of the jaws (36) upon axial movement of the collet (28) relative to the component (24). When positioned on a stem (12, 14) of the valve (10), the cover is movable downwardly thereon to operate the valve and is freely rotatable, but on movement upwardly the cover is clamped by closing of the jaws (36) on the stem (12, 14). The housing (20) is formed of a material with a high degree of resistance to vandalism.