Sieve the flour into a bowl and add the salt, baking powder, Xantham gum and sugar (if used).
Rub the margarine or butter into the flour mixture until it resembles breadcrumbs.
Add the yogurt and lemon juice and mix gently with a spatula until it forms a soft dough.
Add the lemon zest. Combine gently with a spatula and make two ball shapes. Put these in the fridge (wrapped in clingfilm).
Cool for minimum an hour - more if time allows.
Preheat the oven to 180°C.
Roll the dough out between two sheets of parchment paper to 3cm thickness, then stamp out round shapes, using a round cutter or a small mug.
Place the scones on a baking tray lined with baking paper.
Bake them for 12-15 minutes until golden.
Let them cool on a wire-rack, then serve warm or at room temperature.