Preheat the oven to 200˚C, gas mark 6. Line a baking tray with parchment. Put the flour, baking powder, bicarbonate, salt, sugar and butter in a food processor; pulse until resembling fine breadcrumbs (or rub together by hand), and tip into a large bowl. Make a well in the centre, then add the buttermilk, lemon zest and juice, plus most of the blueberries. Gently combine into a shaggy, slightly sticky ball of dough, but be careful not to overwork it, or break up the blueberries.

Generously dust the lined baking tray with flour, then scrape the dough into the centre. Dust the dough with flour, then with floured hands, lightly flatten it into a 20–22cm round. Dust a long, sharp knife with flour and cut the dough into 8 equal wedges, keeping them together in a circle. Dot the remaining blueberries over the top, gently pressing them into the dough. Brush the top with the beaten egg and sprinkle over the demerara sugar.

Bake for 30–35 minutes until golden, firm to the touch and hollow-sounding when the base is tapped. Leave to cool for 5 minutes, then carefully transfer to a wire rack to cool further. Serve in wedges with clotted cream and jam, lemon curd or butter, if liked.

To get ahead: best eaten on the day, but will keep for a day or two. Warm gently in the oven to refresh them.

Freeze: freeze on the day they are made; they can be warmed through in the oven from frozen.