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