Pre-heat the oven to 180ºC. Spray a 1lb loaf pan with low calorie cooking spray and line with non-stick baking paper.
In a bowl add the flour, 2.5 tbsp sweetener, lemon zest, juice of 1 lemon and the egg yolks. Whisk using an electric whisk until smooth and combined.
Add the egg whites to a clean bowl and with clean whisks, mix to soft peaks with an electric whisk. This should take 3-5 minutes. The whites should be thickened and when whisk lifted the peaks should fold over.
Gently fold the egg whites into the flour mixture until mixed evenly. Be careful not to knock out the air.
Scrape the mixture into the lined tin and bake for 25-30 minutes. A knife inserted should come out clean and the cake should be golden brown on top.
While the cake cooks add the remaining sweetener and lemon juice to a small saucepan. Set on a low heat and mix until the sweetener has dissolved.
When the cake is cooked remove from the oven and lower the heat to 100ºC. Pour over the lemon syrup and return to the oven for 10 minutes.
Leave to cool in the tin, slice into 8 and serve.