Ok, so I have now returned to my trailer, fitted the new( or at least old, but rewelded and strenthened) post and removed the roller which I showed a photo of earlier. Once CLH and Swallow Boats had denied any knowledge of this roller I spoke to the previous owner who said he had fitted it, but it hadn't helped and I was best removing it and throwing it away.
Recovering the boat gave me chance to watch exactly what happens when I winch the boat on. This convinced me that the solution I need is the one Graham showed which limits the swing of the swinging cradle, and possibly his other one with the stainless plate bent over the beam. I am now wondering what the best way is of sourcing a suitable method of doing this. Any ideas? Where did you get yours Graham? Has anyone else done it? I suspect best thing will be to find a local welder - again!