Heat half of the oil in a deep ovenproof frying pan, or flameproof casserole dish, and fry the pork pieces over medium-high heat in batches until seared all over, then transfer to a plate. Add another 1 tbsp oil to the pan, if you need to, while you’re cooking the batches. Once all the pork is seared, transfer to a plate and set aside.
Add another 1 tbsp oil to the pan with a little butter and fry half the leeks with a pinch of salt for 10 mins until tender. Add the garlic, fry for a minute, then stir in the flour.
Pour in the cider, a little at a time, stirring to pick up any bits stuck to the bottom of the pan and to combine everything. Add the stock, bay leaves, and seared pork, then simmer, half-covered with a lid for 1-1½ hrs until the meat is just tender (it will later cook to the point of falling apart in the oven). Can be prepared a day ahead.
Heat the oven to 200C/180C fan/gas 6. Simmer uncovered for a few minutes to reduce the sauce, if you need to – it shouldn’t be too liquid or the potatoes will sink into the sauce. Stir in the parsley, chopped sage, remaining leeks, and the cream, then season well.
Peel both types of potatoes and cut them into slices 2mm thick, by hand or using a mandoline. Alternate layers of potato and sweet potato in circles over the pie, or randomly, if you prefer. Dot the cubed butter over the top and bake for 1-1½ hrs until the potato is tender. Nestle in the whole sage leaves, brushed in a little oil, for the last 10 mins. Leave to rest for 10 mins before serving.