Sorry, turns out to be the wrong Helen Beattie. My Helen was born in North Craigo, Angus, not far from Montrose, 1 month before the Helen in the 1921 census so I thought it was my relative. I found out, with help, from a Montrose fb site that the Helen in the census was married to a James Beattie who was at sea at the time of the census.
My grandaunt Helen was born illegitimate to James and Elizabeth (Alexander) Beattie, who married the following year, and was living and working with her mother's sister in Tarbert in 1901...that's the last I can find out about her.