Dear Jack and Jane,
I hope this letter finds you well. I'm just writing to thank you both for the holiday and for the photos you sent.
The photos arrived this morning in the post. They reminded me what a great time I had during my stay with you last month. You really were fantastic hosts, and I couldn't have asked for better guides to show me around.
Sorry I didn't write to you earlier, but I've been working flat out since the moment I arrived home. Do you remember I told you I had an assignment to finish? Well, the deadline was two weeks earlier than I thought it was!
Anyway, I'm back to normal now and I've handed in all of my assignments. In fact, now that I'm free, why don't you both come and stay? There's a spare room here, so you're welcome to use it whenever you like.
Hope to see you soon,
Sincerely yours,