Sift the flour and baking powder together. Beat the egg yolks, lemon zest and juice and sugar together with electric beaters until very light and thick. Sift in the flour and fold in. Whisk the egg whites until firm and then fold into the cake mix without squashing out too much air. Spoon into the tin, level the top and bake for 15-20 minutes until golden brown and a skewer comes out clean. Cool for 10 minutes and then turn out onto another piece of baking parchment dusted with caster sugar. While the cake is still warm, roll it up so it looks like a swiss roll. Cool.