Place the raisins or sultanas in a bowl, pour over the warm tea and allow to soak overnight to plump up. Also, grease and line a 20cm round cake tin with non-stick baking paper. When you are ready to mix the cake, whisk the sugar and eggs together in a large bowl until doubled in volume and pale cream in colour. Warm the olive oil and slowly whisk into the eggs and sugar. Sieve the flour, cinnamon, ginger, bicarbonate of soda, baking powder and a pinch of salt together. Then gradually fold into the oil and sugar mixture until thoroughly incorporated. Drain the raisins from any remaining tea and add to the mixture, along with the lemon zest, diced apples and walnuts. Mix thoroughly. The mixture should be stiff at this stage. Spoon the mixture into the prepared cake tin and even out. Sprinkle over the demerara sugar and bake the oven at 180`c for at least 1 hour, or until a skewer placed in the middle comes out clean. Carefully remove the cake from the oven and allow to cool on a wire rack. Dust with icing sugar just before serving.