My wife and I have tried the Non Cake-Maker's Christmas Cake numerous times over the last year or so and neither of us has been able to prevent the center from sinking badly. Last time I tried I never opened the oven door at all I just turned the oven off and took the cake out when the oven had cooled right down but it still sunk in the middle and on previous occasions I have seen the cake sinking through the oven door. It's a great shame as this is a real favorite with all our family and friends and it would be fantastic if we could present it without the sunken middle. Any suggestions?
Nigella's Non Cake-Maker's Christmas Cake (from Feast) is a type of sponge cake made with store-bought mincemeat (a mixture of dried fruits and cooked apples which can be bought at UK supermarkets and from British food suppliers). The most common reason for cakes sinking in the center is that the cake is not fully baked, or the oven door is opened too early. From the description this does not particularly seem to be the cause, however it is worth checking the oven temperature with an oven thermometer, to make sure that the oven is not running slightly cool.
Make sure also that the leavening agents (baking powder and baking soda/bicarbonate of sod) are not too old as they can become less active over time. They usually have a "best before" date stamped on the packaging. Also measure the leavening carefully as too much leavening can cause the cake to rise up and then sink back during baking. And make sure the oven is fully preheated before you make the cake batter as if it stands for too long before it goes into the oven then the the raising agents will start to expire. Finally, over-beating can cause a cake to sink as it cools. When you add the flour and other ingredients to the butter and sugar try to process or beat as little as possible.