The plot thickens! Having now checked my records at home, it transpires that Harriet Sarah Towell was the mother of Thomas Towell – and the ages that I have for them on my tree match perfectly with those on the passenger lists.
BUT – I originally found these details on the 1891 England and Wales census, conducted some 8 months after their apparent departure to New York.
AND I also have a marriage certificate for Thomas Towell, dated November 1890 for a ceremony taking place in Hackney, London, and showing Harriet S Towell as a witness, and only 3 months after they appeared to be travelling to the States. I think I need to consult with some relatives on this.