Preheat the oven to 180C/350F/gas mark 4. Grease a 21cm x 11cm loaf tin with vegetable oil. In a mixing bowl, cream together the margarine and caster sugar. Now add the gluten free flour, bicarbonate of soda and baking powder.
On top of this lot, pour the lactose free yogurt and the grated zest of one lemon. Whisk everything together.
Juice the lemons. Spoon 2 tbsp juice into a cup and heat the rest in a small saucepan. Add the cornflour to the cup with the juice, then stir until there are no lumps.
Pour it into the pan, whisking constantly. As the mix starts to boil, it will thicken.
Whisk it into the cake mixture and then transfer everything to the cake tin. Bake for 45–50 minutes until the cake has browned and the sides have shrunk away from the tin.
Remove the cake from the oven, but leave it in the tin. While it is still hot, juice the lime into a cup and stir in the granulated sugar (which will not dissolve completely).
Gently prick the cake all over and pour the sugary juice over evenly. Allow the cake to cool before serving.