I think this may be a British film (70's?).

A young woman engaged to an older British man is invited to stay at his home (possibly in England). A boy who is nonverbal secretly collects mementos from her room (e.g., a lock of her hair). She discovers that he is the thief, but realizes that he is not being malicious; he is insecure. I believe the adult son of the woman's fiance is also part of the story.

Thanks for any help on this!