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Unroll the pork and lay it on a board, flesh-side up. Make incisions all over the meat with a sharp knife, then push the slivers of garlic into the incisions. Crush the salt, peppercorns, rosemary and juniper berries roughly using a pestle and mortar, then stir in the olive oil. Rub the seasoning mix into the pork, again on the flesh-side, pushing bits inside the incisions. Put in a dish, cover and chill overnight.
The next day, remove the pork from the fridge and allow it to come to room temperature. Heat the oven to 220C/200C fan/gas 7. Roll the loin firmly, keeping as much of the seasoning inside as you can, then tie it at intervals with kitchen string. Put it skin-side up in a heavy roasting tin and season all over. Roast for 25 mins, then reduce the oven to 200C/180C fan/gas 6.
Pull half the grapes from their stems, cut the rest of the bunch into sprigs, and add them all to the roasting tin with 250ml of the marsala. Continue roasting for 1 hr. Test the meat: the juices should run clear when the flesh is pierced to the centre with a metal skewer (or see tip, below). Remove the pork from the tin, along with the grapes that are on sprigs, and put them on a warmed serving platter. Cover and leave to rest for 15 mins.
Set the roasting tin over a medium heat, add the remaining marsala and crush the loose grapes into the juices. Boil until you have a slightly syrupy mixture. Serve the pork with the marsala sauce and the sprigs of roasted grapes.