Series 3 Episode 5 "Poison" Dr Ormerod says goodnight to Sister Brigid, then walks out of the door from Casualty to the Ambulance Bay. As he approaches the door, his coat is unbuttoned, but when he reaches the other side of the door it is completely done up.
The soundtrack in Season 1, set in the 1960's, included the song "The Air That I Breathe" by The Hollies, which wasn't released until the early 1970's.
Although she is a nun, Sister Brigid clearly has plucked eyebrows and is often shown wearing lipstick--highly unlikely for a woman who has taken the veil.
She appears to be a nurse, not a nun. Somehow the implication that the sisters are nuns got through the gatekeepers.