The IMdb list, though quite extensive, does not include some of her films. She was in John Schlesinger's "Terminus" (1961) - in a small scene although her voice seems dubbed as is the case with everyone in this semi-documentary of a day in the life of Waterloo station. She also appeared as an art mistress in one of the 1950s "St Trinians" films. Schlesinger apparently held her in regard and included her in minor parts in most of his films.
Some years ago Angharad Rees and her then husband took Margaret to Holywood to fulfil a lifelong wish. A documentary was made of this visit which appeared on TV here. Possibly this coincided with her appearance in Magnum.