Thank you for that input, trailers are a world unto themselves.
The way the brackets are attached to the face of the beam, limits the roller to the standard size. I dont know about trying to weld on a longer bracket.
David Rose was concerned about localised deforming of the fibreglass, presumably regardless of the overall internal 'box' strength.
Vern was the one who drew my attention to the problem, and raised my concern, at the Lake Macquarie cruise.
After long thought, Vern's only approach was to raise the whole boat on the trailer, so that there was sufficient room to fit a roller on the last member. He doubted even if this was an achievable solution.
Judging by the info I got with the boat, it has been like this for at least 5 years, if not 10 years.
The Brooker range of trailers appear to rely on the wobble rollers supporting the hull.
If no one has solved this problem, I will take the rather agricultural solution of fiberglassing a wedge (300mm long) on to the skeg, to coincide with the roller.
Apart from a loss in draft, it might be a 'winged keel' in the making?
Careel 18, #421 Gipsy