Pre-heat the oven to 180C/350F/Gas 4.
If the shoulder has been 'rolled', remove the string and flatten out. Using a sharp knife make criss-cross slashes about 2-3cm/0.5-1in apart in the skin.
Lightly grease a baking dish. Lay the pork in it skin-side up.
Peel the quinces then quarter and core, cutting each quarter into two or three slices depending on size.
Scatter the quince slices, onion, chillies, garlic and bay leaves into the baking dish around the pork.
Mix the oil, sugars, wine, vinegar, salt and pepper and spoon over the pork and quinces and onions.
Cover with foil and place in the oven.
Roast for one hour, remove the foil and baste both pork and quinces and onions with their own juices.
Return to the oven, uncovered, for a further hour, basting occasionally.
When the pork is very tender, and the quinces are as soft as butter turn off the oven. Prop the door ajar and leave to rest for 10-20 minutes before carving and serving.
Serve with savoy cabbage and roasted potato wedges.