A mechanism for the adjustment of the vertical alignment of a panel member contained within an elongated channel section comprising; one or more means of clamping the panel member with an adjustable force; and one or more support means for supporting the panel member within the channel section; wherein the one or more means of clamping the panel member is arranged to adjustably tilt and secure the panel member so as to be maintained substantially vertically aligned even when the elongated channel section is secured to a surface which is not substantially horizontal.