I managed to fit a UFO slider as advised. The length is exactly the same as the original, and it has the offset fixings. Where it differs is in the width, and the location arrangement at the front. The wider original has a projection moulded-in that locates in a slot in the arm, whereas the new one has a slot in the rather thin section that wraps around the arm pivot. Unfortunately, making the two slots coincide, perhaps using a peg, would put the slider significantly off the line of the sprocket and chain. I've fixed the slider in line by drilling and tapping a counter-sunk M6 screw on the underside, about 75mm forward of the rear fixing. This should stabilise it sufficiently. When I can get the bike on the road (carburetter troubles!) I shall see how the chain beds into the new slider.
Nick