Pre-heat the oven to 230ºC and dust a baking tray liberally with flour.
Put the Schwartz for Chef Saffron to soak in a splash of boiling water for 10 minutes, then crush with a spoon.
Sieve the flour into a bowl with the bicarbonate of soda, salt and sugar.
Add the butter and rub it in.
Add the walnuts, raisins or sultanas, mixed peel and orange zest.
Lightly beat the egg with half the buttermilk and add to the flour.
Add the saffron and its liquid.
Work together to form a dough, adding as much of the remaining buttermilk as needed to form a soft dough. The less work done on the dough, the lighter the buns will be. Drop the dough onto a well-floured board and pat out into a round about 2cm thick. Do not roll or knead it. Cut into rounds with a 5cm pastry cutter.
Gather up the scraps, press them together lightly and pat out into the same thickness as before. Cut as many rounds as the mixture will yield.
Place on baking sheets and bake for 10-12 minutes, until well risen and brown.