To make the biscuits, preheat the oven to 180°C/Gas 4. Rub the butter into the flour with your hands until the mixture resembles coarse breadcrumbs, then mix in the sugar and lemon zest with a spoon until it forms a stiff dough. Lightly flour a work surface and roll out the dough to about half-a-centimetre thick. Trim the edges and cut into 12 fingers with a knife and put on a baking tray. Bake in the oven for 8-10 minutes until pale golden and remove. Leave to cool on a wire rack. (If, like me, you don't have one of these you can use one of the oven racks instead - the idea is to get some air underneath your biscuits to help them to crisp up.)