To dry the plums, preheat the oven to 100C/200F/Gas ¼. Place the plum halves on a baking tray and place in the oven for two hours. This can be done two days in advance and the dried plums kept in a container in the fridge.
For the pork, preheat the oven to 220C/425F/Gas 7. Open up the pork loin and flatten in on the work surface, fat-side down. Season the plums with sea salt and freshly ground black pepper and place three-quarters of them along the middle of the pork loin. Roll the loin up and secure both ends lightly with skewers then tie the parcel with four turns of string and remove the skewers.
Chop the remaining plums and reserve for the sauce. Heat the rapeseed oil in a heavy-duty roasting pan set over a medium heat and colour the pork bones and trimmings for 7-10 minutes until lightly golden-brown then remove the pan from the heat.
Heat the vegetable oil in a large non-stick frying pan on a medium heat and crisp the skin of the pork loin for 7-8 minutes or until lightly golden all over, then remove the pan from the heat.
Put the bones in a roasting tin and sit the pork loin on top. Add the tomato, garlic and thyme and roast in the oven for 30 minutes. Add the water, and Madeira if using, to create the jus then turn down the oven to 180C/350F/Gas 4, cover the tin loosely with tin foil and cook for a further 35 minutes. Once cooked, add 100ml/3½fl oz of water to lengthen the jus if needed. Place the pork loin on a warmed plate and allow to rest for 30 minutes.
Tip the roasting tin slightly and spoon out half the fat. Strain the remaining jus through a fine sieve into a medium casserole, heat gently then add the reserved chopped plums, season to taste with sea salt and freshly ground black pepper and set aside in a warm place.
To serve, cut away the strings from the pork loin, carve into 8-10 slices and pour the juices released from the meat into the plum sauce. Arrange the pork on a warmed serving dish and pour over the sauce.