Preheat the oven to 180C/350F/Gas 4 and butter a deep springform 18cm/7in cake tin.
Place the flour, sugar and butter in a food mixer with the milk, mincemeat, eggs and lemon zest. Add a pinch of salt and mix on a low speed for a couple of minutes until thoroughly combined. Alternatively, mix with a wooden spoon. Tip into the prepared tin and bake for 30 minutes.
Meanwhile, mix the topping ingredients together in a bowl. Remove the cake and carefully sprinkle the crumble over the top with your fingers. Do it little by little, so that it does not sink into the cake. Return to the oven and continue to bake for a further 50 minutes or until a skewer inserted in the midlle comes out clean (80 minutes baking altogether). Cover loosely with foil for the last 10 minutes or so to prevent burning.
Remove to a wire rack. Leave to cool for about 30 minutes before carefully removing the sides of the tin. Allow to cool completely and remove base if serving cold for tea, or eat barely warm if serving as pudding.