Season 25 g (1 oz) of the flour, then toss the steak and kidney in the flour, shaking off any excess.
Melt 25g (1 oz) of the butter in a large saucepan and lightly fry the garlic, onion and mushrooms for 3 minutes. Add the steak, kidney and remaining coating flour and cook for 5 minutes, until lightly browned.
Stir in the stock, ale, bay leaf, thyme, Worcester sauce and tomato puree. Cover and simmer gently for about 1 1/4 hours. Spoon the mixture into a 1.7 litre pie dish.
Put the remaining flour and a pinch of salt into a bowl. Rub in the remaining butter until the mixture resembles fine breadcrumbs. Add 60ml cold water and mix to form a dough.
Roll out on a lightly floured work surface to 5cm (2 in) wider than the pie dish. Cut a 2.5cm (1 in) wide strip from the outer edge and place on the dampened rim of the dish. Brush the strip with water. Cover with the pastry lid, press lightly to seal the edges. Trim off excess pastry, knot the edges back to seal and crimp. Brush with milk.
Bake at 200 C / Gas 6 for 30 to 45 minutes, or till pastry is a deep golden brown.