Interesting discussion. I guess its just another Morgan foible, and it look's like there are 3 workaround options:-
1) Add zips to facilitate greater manipulation of tonneau material. (Thanks Peter & John for those pictures)
2) Practice the various origami techniques. (Sir Percival - that was a useful link too)
3) Remove tonneau.
Jimmy: I'm glad Madame Fifi helped you on your Morgan journey. I'm sure she would have convinced others too!
Will try some of the suggestions and see how we get on.
Many thanks for the responses.
Mac