A few important things: keep chilling the pastry when you're supposed to, make sure the vegetables are cold or at room temperature when you assemble the pithivier, and don't overfill it - it's tempting.
Cut 425g pumpkin into wedges, peel and deseed these, and cut the flesh into chunks. Spread these out in a small roasting tin. Heat 10g butter with ½ tbsp olive oil, pour it over the chunks, then season. Roast for 15-20 minutes, or until completely tender. Taste the pumpkin. If it is not at all sweet (ie a bland pumpkin), sprinkle a bit of soft brown sugar all over (no more than 1 tsp), then put the roasting tin on the hob and cook, stirring, until the chunks are sweet with golden-brown patches. Spoon on to a baking tray, reserving the fat, and leave to cool completely.
Trim 3 large leeks at the base and cut off the dark-green tips. Remove the coarse outer leaves. Cut into 2cm lengths and wash in a colander. Some of the lengths might fall apart but that's OK.
Put the oil and butter used for the pumpkin in a frying pan. Add the leeks. Cook over a medium heat - adding seasoning - until soft but not coloured. Add 3 sprigs of thyme. If the leeks seem wet, turn up the heat to cook off excess moisture (otherwise it will make the pastry base soggy). Lay the leeks out to cool.
Peel 400g new potatoes or Maris Pipers and slice them wafer-thin, preferably using a mandoline. Heat 300ml double cream, 300ml milk and 1 small garlic clove, grated to a purée, in a pan big enough to hold the potatoes. Add them, season and bring to the boil, then reduce the heat and simmer gently for 15 minutes, or until the potatoes are tender but keep their shape. Remove them with a slotted spoon. It's OK if there is cream and milk clinging to them. Spread the potatoes out on a large tray to cool. Use the leftover milk and cream in a soup (leek and potato would be perfect).
Put 1 block of pastry on a lightly floured piece of baking parchment. Roll it out to 3mm thick and cut out a 26cm circle. Do the same with the other block of pastry but make this circle 28-29cm in diameter. Slide both circles - still on the parchment - on to baking trays and chill completely in the fridge (about 30 minutes). (Wrap up leftover pastry to make cheese straws, or a tart with thinly sliced apples on top.)
To assemble, spread the leeks on the smaller circle, leaving a 2cm rim. Add half the potatoes. Season as you put the layers together, adding grated nutmeg. You need to think of creating a dome shape as you build the vegetables up, not a hill. Lay two thirds of the cheese on top, then the rest of the potatoes. Add the remaining cheese, then the pumpkin. Cover loosely with cling film and chill for 30 minutes.
Brush the rim of the base with egg wash. Take the larger pastry circle from the fridge and put it on top. Firmly press it in place. Scallop the edges of the pithivier, brush it all over with egg wash, then knock up the edges all the way round. Chill for 30 minutes. Brush with egg again and make fine curved lines on the pastry, from the centre to the border. Make a little hole in the top, so steam can escape. Chill for 30 minutes more.
Heat the oven to 200C/190C fan/gas mark 6. Put a baking tray inside. Put the pithivier, still on its parchment, on a baking tray, and slide that on to the hot baking tray in the oven. Bake for 35-40 minutes, or until golden and crisp. Leave to cool for 10 minutes - the cheese will be very hot - then transfer to a warm serving plate.