Trim all excess fat and skin from the whole chicken. Place the meat, water and stock powder in a saucepan and bring to the boil. Cover and simmer gently, about 30 minutes for the breasts or 45 minutes for the whole chicken, until quite tender. Remove the meat from the saucepan.

Strain the stock, make up with milk to 625 ml and set aside.
Remove the meat from the bones, and cut the meat into bite-size portions. Cover and refrigerate until needed.

Place the butter, onion or leek, garlic, mushrooms and peppers in a saucepan and fry very gently over moderate heat until the vegetables are tender but not at all browned. Remove from the heat.

Using a whisk, stir in the cake flour and the stock and milk mixture and bring to the boil, whisking, to form a smooth sauce. Add the meat and season to taste with the sherry, salt and black pepper.

Reheat gently, stirring, until bubbly.
Spoon into a suitable dish for serving.
Or, pour into a suitable dish, cover, cool and refrigerate or freeze immediately.