Butter and line a 500g loaf tin. Put all the cake ingredients in a bowl and beat with an electric beater until smooth. Scrape into the tin and bake at 180c/fan 160c/gas 4 for 45 minutes, until golden and a skewer poked in comes out clean. Leave in the tin.
To make the syrup, gently melt together the juices and sugar. Poke a skewer all over the cake, then pour over half the hot syrup. Cool in the tin, then serve with yoghurt or crème fraîche and the remaining syrup.