Start by making the jam. Place 1 box of blueberries in a pan with 2 tbsp sugar, the lemon zest and juice and 2-3 tbsp water. Bring to the boil and cook gently for 2-3 minutes. Meanwhile, place the flour, baking powder, salt, sugar and margarine in a bowl and rub together to form breadcrumbs. Stir in ½ of the remaining blueberries. Mix the eggs with the buttermilk in a jug (reserve a little to glaze the scones) Add in ¾ of this buttermilk mixture to the flour and mix lightly with a spoon to form a dough, adding in as much more as necessary to form the correct consistency. Roll out on a lightly floured work top and cut into thick rounds. Place onto a baking tray, glaze with the reserved buttermilk and egg and bake in the oven at 200°C for 15-20 minutes. Allow to cool down and serve with the blueberry jam, salted butter and the remaining blueberries