shower screen removal

[essex_gibbo]

i'm trying to remove an old screen. it has the usual wall bracket and two chrome brackets which bolt thro the glass. we have taken off the chrome trim pieces on the fixing screw but the when turning the pozi drive screw the other part just turns so i cannot undo the screw. i have tried an allen key in the hole on theis bit but i cannot lock it. should an allen key be used?? if not any ideas as to how to stop the external part from rotating thus allowing me to ndo the pozi drive screw on the internal part.
thanks
ian