[Gregy]

WOW !! I think you are on to something and that link you sent me was most informative. As far as I knew there was only one 7MM in hex nylon adjustment screw for each light (at the top of the top of the light). But when I look from the top down the side of each light there is another 7MM in hex nylon screw. This is difficult to locate without a torch especially as it was covered in a lot of dust and grime. You would need a very long 7MM hex driver or an extension to reach it, but I did find that a long flat bladed screwdriver will fit into the hex and I was able to turn it.
I will have a play around with this screw one night on the weekend and see if it works.
Many thanks for the information. I will report back how I go !