Thanks everybody. I initially failed to appreciate that the plastic bit came away wih the metal cross beam. once I had sussed out the two long bolts mentioned by Alan it all came together relativly easily. (Though everything was nicely corroded! OxyAcetylene is your friend!)

There was no wiring plug on this one but it was reletivly easy to wire into the existing loom with the owner being my "Eyes" (I am R/G Colour blind, which can be a bit of a PITA at times...)