Make the pastry by rubbing the butter into the sifted dry ingredients (or combine in a food processor until crumbly), add sufficient water to enable the dough to just cling together. Knead gently on a floured surface until smooth, wrap in plastic wrap and put in the fridge for 30 minutes.

Cut the chicken meat from the bone and chop into approximately 2 cm pieces. Melt butter for the filling in a saucepan, add the onion and celery, cook for a few minutes. Add mushrooms and cook for a further minute or so. Add the cornflour, mix in and cook over heat for a minute or so. Remove from heat and gradually stir in the milk until smoothly blended, then stir in the wine. Cook, stirring, until sauce comes to the boil and simmer gently for a minute or so. Remove from the heat, add the chicken pieces, basil, salt and pepper to taste. If the filling is too runny, thicken it by adding a little more cornflour blended with a little milk; if it is too thick add a little extra milk. Transfer the chicken filling into a pie dish (approx 23cm) and allow to cool to room temperature.

Roll out the pastry to fit the top of a pie dish (approx 23 cm), moisten the edges of the pie dish with milk, and place the pastry over the cooled filling in the dish. Press to seal the edges to the dish, trim and pinch the edge decoratively. Decorate with 'leaves' made of left over pastry if desired. Prick the top of the pastry in several places with a fork, brush the top with milk and cook at 400° for about 10-15 minutes, then lower to 350° and cook for a further 15 minutes, or until pastry is golden brown and filling is bubbling.