Preheat oven to 180°C/fan 160°C/gas mark 5.
Cream together the sugar and margarine in a bowl.
Add the Fate low protein all-purpose and the baking powder and rub together with your hands until it resembles breadcrumbs.
In a separate bowl, mix together the ProZero, egg replacer, lemon curd and ½ of the lemon zest.
Add the ProZero mixture to the dry mixture and mix until well combined.
Pour into a lined 20cm cake tin and bake in the oven for 20 minutes or until cooked through.
Remove from the oven and the cake tin and set aside
Whilst the cake is still warm, prick the cake several times with a skewer.
For the topping, mix together the lemon juice and the granulated sugar and pour over the cake. Then allow to cool.
When the cake is cool, mix together the icing sugar and water and pour over the cake.
Top with the rest of the lemon zest and cool in the fridge for 30 mins or until the icing has set.
Fate low protein all-purpose mix and ProZero are foods for special medical purposes and must be used under medical supervision.