The series is based on two novels of Robert Louis Stevenson: "Kidnapped" and its sequel "Catriona". Most of the other "Kidnapped" TV and movie adaptations are based only on the first novel.
The director forgot the need to cast a messenger until everything was set up on location in deepest Gloucestershire. The other actors were hanging around in the cold unable to proceed. Sophie Neville, a young film actress who lived locally, volunteered and quickly changed into an appropriate costume. She was happy to play a boy and used the fee to pay her college fees.