Preheat the oven to 200ºC/400ºF/gas 6. Peel and halve the potatoes, then cook in boiling salted water for 15 minutes, or until tender. Drain, return to the pan and cover. Meanwhile, trim the strip of skin from each chop in 1 piece and set aside. Rub the chops with a little oil, then pick and press a couple of sage leaves on to each side, and season well with sea salt and black pepper. Place a wide ovenproof frying pan over a medium-high heat, add the strips of skin, skin-side down, place a heavy pan on top to keep them flat, then fry for a couple of minutes, or until crisp – just like crackling. Push the crackling strips to the side of the pan, add the pork chops, and fry for 1 minute on each side. Transfer the pan to the oven (if you don't have an ovenproof pan, transfer the chops to a preheated baking tray instead). Roast for 10 to 15 minutes (depending on the thickness of the chops), or until cooked through. Meanwhile, chop half the watercress, pick and finely chop the parsley and trim and finely chop the spring onions. Heat the milk with the bay leaves until almost boiling, then add the parsley, spring onions and the chopped watercress. Switch off the heat and leave the bay leaves to infuse in the milk for a few minutes, then fish out the bay leaves and discard. Add the warm potatoes to the milk and mash until smooth. Season well with sea salt and black pepper. Cover with a lid and keep warm, until needed. Put a little butter on top of the champ, and serve with the pork chops, crackling and remaining watercress.