Heat oven to 200C/180C fan/ gas 6 and line a baking tray with parchment. To make the scones, put the flour, baking powder, sugar and lemon zest in a large bowl and mix together to combine. Add the butter and use your fingers to rub into the flour until the mixture resembles coarse breadcrumbs (don’t rub too thoroughly; some larger lumps are preferable). Add the blueberries and toss together.
In a small jug, mix together the cream, vanilla and eggs, and pour into the mixture. Use a cutlery knife to stir together until the mixture starts to form a dough. Tip onto the work surface and use your hands to gently knead until the dough just comes together. Lightly flour your work surface and flatten the dough into a disc about 2.5cm thick. Use a sharp knife to cut it into eight triangles.
Transfer the scones to the prepared baking tray. Beat the remaining egg, then lightly brush it over the top of each scone. Bake in the oven for about 20-25 mins or until deep golden brown. Will keep for up to 3 days in an airtight container.