Put the chicken, if poaching, in a large pan with the carrot, onion, celery, bay leaf, peppercorns and a few sprigs of tarragon. Cover with cold water, bring to the boil, then skim any scum off the surface. Turn down the heat and simmer gently for about 45 minutes, until the chicken is cooked through.

Meanwhile, start the pastry. Sift the flour and a generous pinch of salt on to a cold surface. Cut the butter into 1cm cubes and stir it in, then gently squidge the two together, so the flour combines with the lumps of butter – the aim is not to mix it completely, so it turns into crumbs, but to have small lumps of butter coated with flour. Like the name, it should look quite rough, even unfinished.

Sprinkle a little of the water over the top and stir it in. Add enough water to bring it into a dough (unless your kitchen is very dry, you probably won't need it all), without overworking the mixture, then cover with clingfilm and refrigerate for 20 minutes.

Lightly flour a work surface and shape the dough into a rectangle. Roll it out until three times its original length. Fold the top third back into the centre, then bring the bottom third up to meet it, so your dough has three layers. Give the dough a quarter turn and roll out again until three times the length, fold again as before, and chill it for 20 minutes.

Remove the bird from the pan and set aside. Turn the heat up and reduce the stock by about three quarters to about 500ml – this should take about 20 minutes. Add a good slug of madeira or sherry, and season to taste. Once the chicken is cool enough, strip off the meat.

Preheat the oven to 180C/350F/gas mark four. Melt the butter in a large frying pan over a medium heat and add the lardons. Cook until golden, then scoop out of the pan and set aside. Add the leeks to the pan and fry for about seven minutes until silky, then turn the heat up and, once the pan is hot, add the stock to the pan; it should sizzle. Allow to bubble for a minute or so, then stir in the cream, lemon zest and the leaves of the remaining tarragon, roughly chopped, plus the bacon. Season to taste, then stir in the chicken.

Pour the chicken and sauce into a large pie dish and allow to cool. Roll out the pastry on a lightly floured surface to about 5mm thick. Brush the rim of the pie dish with egg wash, then place the pastry on top, pressing down to seal. Crimp the edges if you like, then cut a small hole in the top and brush the whole lot with egg wash.

Bake for about 30-40 minutes until the pastry is risen and golden, and allow to cool slightly before serving.