Mix the flour, baking powder, sugar and salt together in a large bowl. Make a well in the centre, then pour in the milk and lemon juice, then crack both eggs in. Whisk together until they form a smooth but fairly thick batter.
Melt the butter in a large non stick frying pan, then pour the batter in palm-sized circles into the pan. After a couple of minutes, bubbles will start to appear in the batter on the top of the pancake and the underneath will be golden brown. At this point you can (carefully!) flip them.
Meanwhile, add the blueberries, sugar, lemon zest (finely grated) and juice to a small pan over a low to medium heat, stirring until the sugar has dissolved and the blueberries start to split, creating more of a sauce. Whisk in the butter and remove from the heat. (Some leave it to cool before serving...I don't!)