Yeah I think the informant would be the most reliable you could get, but I would feel a bit awkward contacting them I think. I guess her infomation would only be as reliable as what Sheila herself was told!
I remember when I was searching BMDs there was one I discounted. I can't access ancestry to check for definite but I think it was something like:
Joan or Sheila MCDONALD
registered 1913?
Mother's maiden name: Boud
And was somewhere in Essex.
I might order that cert if I get no joy with the Scottish one! Although that really is clutching at straws a bit. I think there was a son registered a couple of years later and as far as I know Sheila didn't have a brother
