The diagnostics prize goes to Jon Tilson.
Once the car was on the hoist and lit underneath by a bright light, the shiny mark on the prop shaft was obvious. The front silencer had been rubbing on the prop shaft because the front exhaust bracket (the one that anchors the joint between the front pipe and the intermediate pipe to the rear of the gearbox) had given way. Under power the engine twists slightly on its mountings and maintains clearance. Decelerating, the engine twists slightly in the opposite direction and the clearance between exhaust and prop shaft reduces to almost nothing. Turning left while decelerating causes centrifugal force which causes the exhaust to swing outwards slightly, and just enough to cause the front silencer to rub on the prop shaft. By replacing the exhaust mounting, the problem went away.
Many thanks for all the suggestions of what to look for. It has been an education
Jim