Mix together the flour, sugar, baking powder, salt and xanthan gum in a mixing bowl. Pour in 250ml warm water and the olive oil into a well in the centre of the mixture. Quickly combine the ingredients to create a thick, paste-like dough. Add a small amount of warm water if the dough feels dry. Put the dough in a bowl, cover with cling film and put in the fridge for no more than 24 hours before using. Lightly flour two baking/pizza trays. Cut the dough into two pieces and flatten into thin rounds, roughly the size of a dinner plate. Add to the baking/pizza trays.