Heat oven to 220C/fan 200C/gas 7. Put the lamb into a large ovenproof dish. Pour the oil into a small food processor with the oregano and rosemary leaves and the garlic, then whizz to a rough paste. Season the paste well, rub all over the lamb, then roast for 20 mins. Cover, lower the oven to 150C/fan 130C/gas 2, then roast for a further 3 hrs.
Remove the lamb from the oven and carefully pour off all the fat, leaving any meat juices in the dish if you can. Pour over the wine and tomatoes, poke in the remaining herb sprigs, then return, uncovered, to the oven for a further 40 mins. The lamb should now be tender enough to cut with a fork or spoon.
Carefully pour or spoon the wine and tomato sauce into another pan, skimming off any fat that rises to the surface, then re-cover the lamb. Let it rest for up to 30 mins while you roast the potatoes and finish the sauce. Heat the tomato mixture until bubbling, then simmer for 10-15 mins until thickened and saucy. Season with the sugar, and some salt and pepper if it needs it, then pour back around the lamb to serve.