Dear Mrs Smith,
I can't tell you how much I appreciated your kindness in letting me stay at your home during the past month. Your family is wonderful, and I couldn’t have felt more welcome. I truly enjoyed the time I had in England, it was an unforgettable experience.
I am writing to ask you a little favour. I'm affraid I have accidentally forgot to take my diary when I left your house. As far as I remember, it is in my room on the desk. This diary means a lot to me. I'd be grateful if you could send it to me as soon as possible. I hope that this does not cause you any inconvenience, but please contact me if it is a problem.