Put the pork on a board and cut off the string if your pork has been rolled and tied. Slide a large sharp knife under the pork rind and fat, then lift it off completely. Set it to one side. Cut a hole about 4cm wide through the thickest part of the loin, or eye, of the pork. Preheat the oven to 190C/Fan 170C/Gas 5.
Finely chop the sage and parsley leaves and put them in a bowl. Add the lemon zest, garlic, salt, and plenty of freshly ground black pepper. Press some of this herby mix into the hole in the pork and then spread the rest over the surface of the pork until well covered.
Cover the meat with the pork rind - this will protect the pork and keep it moist as it cooks. Tie the joint at 2cm intervals with kitchen string to keep it together and place in a small sturdy roasting tin. Roast for 30 minutes per 500g, plus 20 minutes. A 1.25kg joint should take 1 hour 35 minutes, but check it is cooked throughout.
When the pork is ready, move it to a warmed board ready to carve and cover it with foil and a couple of dry tea towels to keep it warm. Holding the roasting tin carefully with an oven cloth, tilt all the juices to one corner and skim off as much of that naughty fat as possible with a spoon and throw it out.
To make the gravy, put the roasting tin on the hob over a medium head and add the apple juice and 50ml of the cold water. Stir well to lift the juices and sediment from the bottom of the pan. Simmer for a couple of minutes. Carefully strain the gravy through a sieve into a small saucepan and bring it back to a simmer. Mix the cornflour and the remaining cold water to make a thin paste. Stir this into the gravy and cook for 1-2 minutes more until thickened, stirring occasionally. Adjust the seasoning to taste.
Remove the string from the pork and lift off the crackling and cut it up separately. Carve the pork and serve it without the crackling for anyone watching their weight! Pour the gravy into a jug to serve with the pork.