Preheat the oven to Gas Mark 7, 220°C, fan 200°C. Mix the garlic, lemon zest and fennel seeds to make a thick paste.
Unroll the pork and pat dry. Score the fat, making cuts about 1cm (½in) apart, then turn flesh side up and rub with the paste. Roll back up and secure with string. Rub the skin with sea salt and place on a rack over a roasting tin.
Cook for 20 minutes, then reduce to Gas Mark 4, 180°C, fan 160°C, and cook for 1 hour and 15 minutes (or 25 minutes per 500g (1lb). For crispy crackling, increase to Gas Mark 7, 220°C, fan 200°C for the last 20 minutes of cooking.
Meanwhile, mash the boiled potatoes in a bowl with the butter and mik. Season.
Leave the pork to rest. Put the roasting tin on the hob and spoon off the excess fat, leaving the juices. Heat gently, then stir in the flour and cook for 1 minute, whisk in the stock and bring to the boil. Simmer for 5-10 minutes, until thickened, then strain into a clean pan. Reheat, adding the lemon juice and season to taste. Serve with the pork, mash and some green beans.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.