To draw a line under this I stuffed a random bit of brass about 1.5mm thick between the base and the arm just to see what would happen:

I took a few of slices off a 50mm steel bar and the error seems to have gone down from ~2mm to ~60 microns over 50mm I can't believe that either, but it does seems to be cutting square now. Lucked out perhaps. I suspect that this is one of those lash-ups which will become permanent!
Eric – thanks for the link. Interesting reading, but not directly relevant to my problems. The Axminster saw has no provision for adjusting blade tracking, tension, guide angles or anything else. As MichaelG suggested they should have made it right or made it adjustable. In this case they did neither.
Robin.