Sift flour and salt into a bowl and leave in a warm place. Dissolve yeast and sugar in 1/4-pt of the warm milk and water. Leave to froth, then mix in the fat. Stir all the liquid into the warm flour and beat well until smooth and elastic. Cover and prove in a warm place for 50 minutes or until doubled in bulk. Turn onto a well-floured board and knead, working a little more flour if necessary to make the dough easier to shape.
Round up the dough, roll into a thick sausage shape and (using the sharpest knife you have) slice into 8 to 10 portions, each about 1 1/2 to 1-3/4 inch thick. Shape each one into a round with straight sides. Put onto a greased baking sheet. Cover (and put in a warm place to prove for 30-40 minutes or until springy to the touch. Leave room for expansion and be careful not to over-prove, as the muffins will get flabby and lose their shape. Warm and grease the bakestone lightly.
Lift the muffins carefully onto the bakestone and cook over very moderate heat for 8-10 minutes until pale gold underneath. Turn and cook the other side. Wrap in a cloth and keep warm if cooking in batches. To serve, insert a knife in the side, pull the top and bottom slightly apart, and insert slivers of butter.