Dry the skin of the duck thoroughly with kitchen paper, inside and out. Prick it all over with a fork to help reduce fat during cooking. Cut off the wing tips for the stock.
If you can, leave for a few hours in a cool dry place to dry further, or uncovered in the fridge overnight.
Preheat the oven to 230 c, Gas 8. Rub the duck with a dessertspoon of fine sea salt and lay on a rack in a roasting tin, breast side down.
Roast at a high temperature for 15 minutes, then turn over and roast for a further 15 minutes or until the skin is a light mahogany colour and starting to crisp.
Turn the oven down to 180 c, gas 4 and cook for a further 90 minutes.
Meanwhile, make a stock from the giblets and wing tips of the duck, the stock ingredients and enough water to cover by 2 cm. Strain and skim off any fat and reduce the stock for about half an hour to 200ml.
When the duck is cooked remove to a board and leave to rest, loosely covered with foil. Pour off 95% of the duck fat from the roasting tray and reserve for roast potatoes. Pour any juices from the cavity of the duck into the tray.
Put the roasting tray over a low to medium heat and add the sherry. Simmer for a few minutes then add the duck stock and membrillo. Melt the membrillo and reduce for another few minutes. Season.
Quarter the duck and serve with Moros y Cristianos and the warm membrillo sauce.