It's a terrible problem having one of these pies hanging around. Sitting there in the fridge it becomes a delicious diversion from work. Stupidly small slices demand an endless succession of sneaky trips to the kitchen until someone shouts: 'Hey, who ate all the pie?' The only true place for this is on a picnic, where the whole lot gets eaten in one go. A lone picnic, of course.
Cut each chicken breast in half lengthways and put in a large bowl. Cut the sliced ham into smallish pieces, about the size of an ordinary postage stamp. Trim the leeks and cut into roughly 3cm lengths. Add the ham and leeks to the chicken along with the diced lard, ginger, mace, salt and ground black pepper. Muddle together well, then put to one side while the pastry is made.
Grease a 17.5cm spring-clip cake tin with lard. To make the pastry, sift the flour into a large bowl and stir in the pepper. Rub in the butter and 50g of the lard until the mixture resembles fine breadcrumbs. Make a well in the centre. Put the water and salt in a small saucepan. Bring to a simmer, then remove from the heat and whisk in the remaining 125g lard. Pour this into the bowl with the flour. Stir well, first with a wooden spoon, then with your fingers, to make a smooth, pliable dough. Cover and leave for 15-20 minutes to allow it to become cool and workable.
Working quickly, reserve a third of the pastry to make a lid and roll out the remaining pastry on a heavily floured work surface until around 5mm thick. Use this to line the cake tin, leaving around 3cm overhanging the edge. Press well into the base and side of the tin. Be very careful not to rip the pastry, as this will make it impossible to pour in the jelly later. Spoon the chicken mixture into the pastry-lined tin, ensuring that the leeks and ham are dotted pretty evenly throughout. Don't pack everything in too tightly, as the jelly stock you'll be pouring in must be able to sneak its way round the entire interior.
Roll out the remaining pastry into a roughly 24cm round. Brush the overhanging edges of the pie with water and place the pastry lid on top. Press the edges firmly together to seal, then trim off most of the excess, leaving just enough to pinch into a neat crimped edge. Make a 1cm hole in the centre of the pastry lid. Place on a sturdy baking tray and chill for 30 minutes.
Preheat the oven to 180°C/350°F/Gas 4.
Bake the pie in the centre of the oven for 1 hour. Take it out of the oven, very carefully release the sides of the tin and remove, but leave the base. Take great care not to crack the pastry, as this will be disastrous when it comes to pouring in the jelly. Return to the baking tray. Brush liberally with the melted lard for the glaze, then with some of the beaten egg, and continue baking for 15 minutes. Take out of the oven and brush with more egg. Return for a further 15 minutes until the pastry is crisp and golden brown. Leave to cool for 2â€“3 hours, putting in the fridge after the first hour.
To make the jellied stock, soak the gelatine sheets in a bowl of cold water for 5 minutes until softened. Heat the chicken stock in a small saucepan until it has just boiled, leave to cool slightly, then taste and tweak the seasoning, if needs be. Squeeze out the gelatine over the bowl, getting rid of the excess water, and drop it into the warm stock. Stir until completely dissolved, then transfer to a jug and leave to cool for 30 minutes. Do not allow it to set. Using a funnel, pour the stock slowly into the pie, stopping every so often to wait for the liquid to trickle down between the layers. Halt when the stock reaches the top. Chill the pie for several hours or overnight until the stock jellifies. Serve the pie in wedges with English mustard and chutney.
NOTE: If you can't get hold of baby leeks, use the slimmest leeks you can find. You'll need about 400g prepared weight.