Heat the oven to 180°C (160°C in a fan oven), 350°F, gas 4.
Season the meat well with salt and ground black pepper and brown all over in hot oil in a roasting tin. Arrange the garlic cloves and rosemary around the meat, pour over the wine and stock and cook in the oven for around 1.5 hours, basting from time to time and adding extra water if needed.
Take the joint out of the tin and wrap it in foil. Put the meat in the oven to keep warm, first making sure that the oven is turned off.
Heat the roasting juices on the hob and add the sugar and the blackcurrants, putting 1 tbsp of blackcurrants to one side for later. Simmer for around 10 minutes, adding a little stock if necessary.
Press through a sieve and capture the sauce in a small pan. Mix the cornflour in a little water and add to the sauce. Return to the boil and season to taste with salt, ground black pepper and the blackcurrant jelly. Stir in the reserved blackcurrants.
Fry the spring onions in the olive oil for 3-4 minutes until browned on all sides, and season with onion salt and white pepper.
Remove the roast pork from the oven, slice and arrange on a warmed serving plate. Spoon over the sauce and sprinkle with parsley. Serve with the spring onions, and a helping of rosemary potatoes if desired.