Castaway (1986)

"One Year on a Deserted Tropical Island. 'Wife' 20-30 needed to accompany man 35+. Write to Box 775 with details and evening phone number."

Middle-aged Gerald Kingsland advertises in a London paper for a female companion to spend a year with him on a desert island. The young Lucy Irving takes a chance on contacting him and after a couple of meetings they decide to go ahead. Once on the island things prove a lot less idyllic than in the movies, and gradually it becomes clear that it is Lucy who has the desire and the strength to try and see the year through.