OK - I am a complete and utter dunce
Having re-examined the light fitting I discover that there is indeed a screw holding one end of the fitting together as HLaB suggested.
It has been so long since I fitted it that I forgot that undoing the screw would open up the bracket.
In my defence I would point out that I did show it to staff at two separate LBSs, and they both said that they only way of getting it off was to cut though the bracket which seemed a bit drastic to me. Hence my original post.
Having re-examined the light fitting I discover that there is indeed a screw holding one end of the fitting together as HLaB suggested.
It has been so long since I fitted it that I forgot that undoing the screw would open up the bracket.
In my defence I would point out that I did show it to staff at two separate LBSs, and they both said that they only way of getting it off was to cut though the bracket which seemed a bit drastic to me. Hence my original post.