Inspired by the GBBO I have tried a number of (usually) Mary Berry recipes and all looked good, tested done and then sank in the middle as they cooled. Suggestion that too much baking powder though UK and North American self rising four all say one and a half teaspoons of baking powder per cup of flour. I wonder whether anyone else is experiencing this problem? Bread recipes come out just fine, btw.
(Oh, and $10 off your order of $50 or more in the Food52 Shop, too.)