Heat oven to 180C/fan 160C/gas 4.
Beat together 225g softened unsalted butter and 225g caster sugar until pale and creamy, then add 4 eggs, one at a time, slowly mixing through.
Sift in 225g self-raising flour, then add the finely grated zest and juice of 1 lemon and mix until well combined.
Line a loaf tin (8 x 21cm) with greaseproof paper, then spoon in the mixture and level the top with a spoon.
Bake for 45-50 mins until a thin skewer inserted into the centre of the cake comes out clean.
While the cake is cooling in its tin, warm up the lemon curd in the microwave for about 30 seconds.
Prick the warm cake all over with a skewer or fork, then pour over the lemon curd.
Leave in the tin until completely cool, then remove and serve. Will keep in an airtight container for 3-4 days, or freeze for up to 1 month.