A small grammatical point:  I think Rastus' should be Rastus's, because it is not plural.

A Very Dear Cat

Mrs. Ramsay received an anonymous letter three days after her cat Rastus’ disappearance. The kidnapper demanded 1000 pounds for the return of the cat. She would never see Rastus if she went to the police. She was instructed to place the money in a cardboard box and put it outside her door. She withdrew 1000 pounds from her bank and acted on the kidnapper’s instructions. The money had disappeared the following morning and Rastus was returned at seven o’clock that evening.

Just one word that needed correcting, looks like a typing mistake.

Nice, short sentences are great! People love commas too much, but not you. :-)