Preheat the oven to 180ºC/350ºF/gas 4, and grease and line the base of a 23cm square cake tin.
Core and coarsely grate 2 of the apples and finely slice the remaining apple.
Melt 85g of the sugar and the margarine in a pan, then pour into the prepared tin. Top with the sliced apple in a single layer.
Combine the flour, 110g of sugar, the bicarbonate of soda and mixed spice in a bowl. In a separate bowl, combine the oil, 180ml water, the vinegar, grated apple and lemon zest. Mix the dry ingredients with the wet, quickly but thoroughly.
Roughly chop and stir in the walnuts, then pour over the layer of apples in the cake tin.
Bake for 30 minutes, or until a skewer comes out clean. Leave the cake to cool for 5 minutes before turning out.