Preheat the oven to 220oC/Fan 200oC/ Gas Mark 7. Reserve a few small sprigs of watercress to decorate the tops. Finely chop the remainder.
Sift the flour, baking powder and mustard into a large bowl. Add the salt, butter, then use your fingertips to rub it into the dry ingredients. Stir in the chopped watercress and two thirds of the cheese, mix well.
Make a well in the centre of the dry ingredients, add the buttermilk and mix together with a round bladed knife to form a soft dough.
Very lightly knead on a floured surface, then use a floured rolling pin to roll the dough to a thickness of 2.5cm. Use a 6cm plain round cutter to press out circles of the dough, re-rolling lightly, as necessary. The mixture should make 9 scones.
Place the scones on an oiled large baking sheet. Brush each with a little buttermilk (or milk), top with a sprig of watercress, then scatter over the remaining cheese. Dust with a little pinch of cayenne if liked, then bake in the centre of the oven for 20-25 mins or until golden on the top. Leave to cool on the baking tray for 5 mins before serving warm.