Preheat the oven to 200°C, fan 180°C, gas 6. Mix the garlic with the oregano, rosemary, lemon zest and juice and olive oil in a small jug.
Using a small sharp knife, cut slits all over the lamb and place in a roasting tin. Drizzle the herb mixture over the lamb and, using your hands, work as much as possible into the slits in the meat. Roast for 20 mins.
Reduce the oven temperature to 180°C, fan 160°C, gas 4 and cook the lamb for a further 1 hour 15 mins for a 1.9kg joint or 20 mins per 450g for pink lamb; cook for a further 15 mins if you prefer it well done.
Add the potatoes to the roasting tin when the lamb has 1 hour left to cook. You will need to baste the lamb once and turn the potatoes to make sure that they brown evenly.
Take the lamb out of the oven and transfer to a platter, cover with foil and leave to rest somewhere warm for 10–15 mins. Spoon the potatoes into a serving dish and keep warm.
Pour the fat off the juices in the tin, then put the tin on the hob. Add the tomato passata, 200ml hot water, the olives, brown sugar and rosemary. Simmer gently for a few mins.
Carve the lamb and serve with the potatoes, vegetables and the sauce for the adults.