Polarbearflavour This is a question that has exercised me a great deal! As you say, Sister Julienne definitely existed, as did Cynthia, though her story wasn't the story they've gone with in the series. I believe she died of cancer a few years before Jennifer Worth, but they remained close friends throughout her life.
I think Trixie may have been real or an amalgam.
When the Guardian did an interview with the remaining Nonnatus nuns a few years ago, they couldn't recall anyone who resembled either Monica Joan or Chummy.
Chummy is interesting, though, becuase Jennifer Worth certainly spoke about her as if she was a real person, and specifically asked for Miranda Hart to play her, because she reminded her of the real Chummy. And I remember reading in the Radio Times that JW's daughters gave the production team a picture from Nonnatus House of the midwives that included Chummy, so that they could have an idea of what she looked like.
To my knowledge, though, that photo has never re-emerged, and details in the books about Chummy apparently couldn't have been true (e.g. the nursing prize she supposedly won didn't actually exist).
So, who knows whether she ever existed or whether JW just invented her? Shelagh: where did you see that she'd admitted to Heidi Thomas that she made most of it up?
I feel a bit troubled by all of this. The books read like social documentary - in fact, bits of them are quoted in David Kynaston's series of books about postwar Britain, clearly on the assumption that they are truthful. They made for harrowing reads, principally because one assumed that these were real-life stories - Jane's being a prime example.
How much did she write that was true, and can we ever know? I remember reading that James Herriott turns out to have made up most of the stories in the Vet books, and I wonder if this is the same.