Preheat the oven 200c conventional oven,180c fan, or Gas mark 6. Line a 2lb loaf tin with greaseproof paper and set aside. Melt the butter on a low heat until just creamy, then remove from the heat and pour into a large mixing bowl. Beat in the sugar, zest and eggs, then fold in the self-raising flour. Add a splah of milk and fold it in along with the blueberries, then pour into the prepared tin. Bake in the centre of the oven for 60-80 minutes, or until a skewer inserted into the cake comes out clean. Remove from the oven and leave to cool in the tin for 20 minutes before turning out onto a wire rack to cool fully.