If your lemon drizzle cake is sinking in the middle, it could be a few issues. Firstly, you must never open the oven door too soon or your cake will sink. Always leave it at least 50 minutes before opening the door. Secondly, try to get the cake batter in the oven as soon as it's mixed - and always make sure the oven is fully preheated before putting the cake in. This is why I always list preheating as the first step, so the oven is nice and hot by the time the cake is ready to bake.