That wouldn't surprise me in the least - public life was still very formal in Britain during Margaret's lifetime and she was very conscious of her place in the family as The Queen's only sibling. It's only been in the past few years that The Queen's grandchildren have started to refer to her as "granny" in public - and that has only been Harry, Beatrice, and Eugenie. Prince Charles will occasionally refer to her informally as "mummy" but only for effect - like at the Jubilee. As we saw in the documentary he made for her 90th he personally calls her Mama - with the emphasis on the second syllable. Anne, Andrew, and Edward refer to her as "Her Majesty" or "The Queen" in public as do Sophie, Peter, Zara, and even William. Formality still reigns in public.
I remember someone, I think it was James Whittaker, saying he was talking to Princess Anne when the butler came in to say the Queen was on the phone, Anne stood up the entire time she was talking to the Queen and only sat back down when she finished. He was made the point Anne was being respectful to HM and did it without even realising, that how ingrained into her it was. I wouldn't be surprised if to members of the public Margaret expected her children to refer to her as HRH