Line a 20cm cake tin with greaseproof paper or baking parchment, Preheat the oven to 175°C (fan) / 350°F / Gas Mark 4.
In a food mixer or large bowl, whisk together the margarine and soft brown sugar for 2-3 minutes, until fairly pale and fluffy.
In a small jug or bowl, combine the vegan yoghurt and milk, cinnamon and lemon zest.
Peel and core two of the apples, and cut them into chunks. Core the third apple (but don't peel) and cut it into thin slices.
Tip the milk and yoghurt mix into the cake mix, then sieve the flour and baking powder into the mixture. Mix gently until just combined
Fold through the oats, apple chunks (leave the slices for later) and sultanas.
Tip the mixture into your baking tin and smooth out the top.
Arrange the apple slices on top of the cake mix in whatever pattern you choose.
Sprinkle with the demerera sugar, then bake for 60-80 minutes until a skewer comes out completely clean, (NB a slightly larger tin = shallower cake, so it will cook more quickly). If the apple slices on top seem to be browning too quickly, cover the cake with foil for the remaining cooking time.
Leave to cool in the tin, then lift out and slice.