Place the flours, xanthan gum, baking powder, bicarbonate of soda and butter in a food processor and whiz until the mixture resembles fine breadcrumbs, or rub in by hand in a large bowl.
Stir the olives, tomatoes and parsley into the mixture, then, using the blade of a knife, stir in the egg and buttermilk until the mixture comes together.
Tip the dough out on to a surface dusted lightly with flour and gently press it down to a thickness of 2.5 cm (1 inch). Use a 5 cm (2 inch) cutter to cut out the scones.
Place on a lightly floured baking sheet, brush with a little buttermilk and place in a preheated oven, 220°C (425°F), Gas Mark 7, for about 12 minutes until golden and risen. Remove the scones from the oven and transfer to a wire rack to cool.