Preheat the oven to 160C/320F/Gas 3.
For the Yorkshire pot, butter a large piece of kitchen foil. Wrap the Yorkshire pot in the buttered foil to cover completely.
Place the leek halves into a roasting tin and place the foil-wrapped Yorkshire pot onto the leeks. Transfer to the oven and roast for 2½ hours.
Remove the Yorkshire pot from the oven and remove the foil.
Increase the oven temperature to 200C/400F/Gas 6.
Return the Yorkshire pot to the oven and cook for a further 10-15 minutes, or until golden-brown all over and completely cooked through. Check that the birds are cooked by placing a knife into the centre - the juices should run clear. If they are at all bloody, return to the oven and cook until the juices run clear. Remove from the oven and rest for 15 minutes before serving.
For the roast potatoes, keep the oven temperature at 200C/400F/Gas 6.
Place the potatoes into a saucepan and cover with water, and add a pinch of salt. Bring to the boil and cook for 3-4 minutes, then drain into a colander.
Place a deep roasting tray onto the hob and add the lard or dripping. When the lard or dripping has melted, add the potatoes and fry in the roasting tray until the potatoes start to brown. Turn occasionally to ensure the potatoes brown on all sides.
Sprinkle the potatoes with salt and place into the oven to roast for one hour, turning occasionally to ensure they cook evenly.
For the bread sauce, place the clove-studded onion and milk into a heatproof bowl set over a pan of simmering water. Heat the milk until just below boiling point (the longer the milk takes to come to boiling point, the better the flavour of the sauce will be, so don't rush this process).
Remove the onion and cloves from the milk, but keep the milk over the simmering water.
Gradually add the breadcrumbs to the milk, whisking constantly, until the sauce becomes thick.
Season the sauce, to taste, with freshly grated nutmeg, salt, white pepper and cayenne pepper.
Add the butter and cream and stir well until the butter has melted. Pour the sauce into a serving bowl and keep warm.
For the cranberry and apple jelly, place the cranberries and apples into a saucepan with the water.
Add the cloves and cinnamon and bring to the boil. Reduce the heat and simmer for 10-15 minutes, until the cranberries have split and the apple has become tender.
Using a ladle, gradually strain the cranberry and apple mixture through a muslin-lined sieve into a large clean bowl. Don't press or force the berries as this will make the jelly cloudy.
Add the sugar and vinegar to the liquid and mix well.
Transfer the liquid to a clean pan and heat to just above boiling point - 104C on a sugar thermometer. Remove from the heat and leave the mixture to cool, then transfer to a sterilised jar.
For the sprouts and pancetta, bring a saucepan of salted water to the boil and add the sprouts. Cook for 8-10 minutes, until tender, then drain into a colander.
Heat a frying pan over a high heat until hot. Add the olive oil and pancetta cubes and fry for 5-7 minutes, until the pancetta is crisp and golden-brown.
Add the drained sprouts and stir together to warm through.
To serve, carve the Yorkshire pot carefully to ensure that each slice contains a portion of each bird. Place a couple of slices onto each plate. Spoon the roast potatoes alongside, then add a spoonful of the sprouts and pancetta. Serve the white sauce and cranberry and apple jelly alongside.