Heat oven to 180 C /160 C fan and grease and line a 2 lb loaf tin with greaseproof paper and butter.
Beat together the butter and caster sugar in a stand mixer until pale and creamy, about 5 minutes, then add the eggs, one at a time, slowly mixing through. If the mixture looks like it’s about to split, add a tablespoon of flour to bring it back together.
Sift in the self-raising flour and add the finely grated lemon zest and the lemon extract/oil then fold in until well combined. If the mixture is too thick, add 1 tablespoon milk to loosen slightly.
Spoon the mixture into your prepared tin and level the top with a spoon or offset spatula.
Bake for 30 mins, uncovered, then cover with foil and bake for a further 30 minutes, or until a skewer inserted into the centre of the cake comes out clean.
Whilst the cake is cooking, mix together the lemon juice, icing and caster sugars to make the drizzle and set aside until needed.
Once out of the oven, prick the warm cake all over with a thin skewer, then immediately pour over the drizzle – the juice will sink in and the sugar will form a lovely, crisp topping.
Leave in the tin until completely cool, then remove and serve with a little lemon zest grated over the top if you wish.