Preheat the oven to gas 7, 220°C, fan 200°C. Crush the chilli, garlic, and half the rosemary in a pestle and mortar with a good pinch of sea salt and black pepper. Grind together to form a rough paste, then add the fennel seeds.
Make 1cm (½in) deep slits in the lamb, then rub the chilli paste over the meat, pushing it into the slits. Drizzle with oil and season. Scatter the remaining rosemary over a large roasting tray and sit the lamb on top. Put the lamb in the preheated oven.
After 20 minutes reduce the oven tempurature to gas 6, 200°C, fan 180°C. Add the wine and a splash of water and then roast for 1 hour 15 minutes (or 15 minutes per 500g) for medium, or 1 hour 35 minutes (or 20 minutes per 500g) for well done meat. Baste the lamb a couple of times during cooking, then, about 15 minutes before the end, pour a cup of water into the tin, baste again and return to the oven.
Remove the lamb from the tin, transfer to a serving plate and leave to rest for 20 minutes. To make the gravy, spoon off any excess fat from the juices, then put the tin over a high heat on the hob. Scrape any sticky bits from the bottom of the tin with a wooden spoon, then add the flour and whisk. Add the stock and leave to bubble for 3-4 minutes. Stir in the jelly, season and add a little more water or stock to reach the desired consistency. Serve thick slices of lamb with the gravy.
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