Put the pork in a roasting tin and brush with a little oil. Sprinkle with salt and roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes. Reduce the oven temperature to 180°C (350°F), Gas Mark 4, and roast for another 1 hour or until crisp and golden. Test to make sure that the pork is cooked through by inserting a skewer into the meat – the juices should run clear. Set aside in a warm place, covered with foil, for 15 minutes.
Make the sauce, meanwhile. Pour the stock into a saucepan. Add the tomato purée, orange rind and chilli flakes. Bring to the boil, then reduce the heat and simmer for about 30 minutes until the sauce is thick.
Take off the crackling from the roast pork using a sharp knife and cut into strips. Next, cut away any fat on the pork and discard. Tear the meat into shreds. Add the shredded pork to the sauce and heat over low heat.
Pop the tortillas in the oven for a couple of minutes to warm and soften (don't let them become dried out and crisp, or you will not be able to roll them properly). Put a little of the pork mixture in the centre of each warm tortilla and roll them up. Dollop soured cream on top of the burritos, then pile on the avocado and sprinkle chilli flakes over the top. Serve with the crackling, if you wish.