Preheat the oven to 200C/400F/Gas 6. Remove the lamb from the fridge and allow it to get to room temperature.
Make the garlic and rosemary butter. Peel the garlic cloves and grate them finely with a grater. Place into a bowl with the butter.
Strip the leaves off the rosemary stalks and chop finely. Add to the bowl with the garlic and butter. Season with salt and freshly ground black pepper and mash to a paste with a fork.
With a sharp thin knife or a metal skewer, make between 30 and 50 small incisions into the flesh of the lamb. They should be large enough for you to stick just your fingertip into.
Work the butter and garlic mixture into the lamb by massaging it into the meat, trying to work it into the holes that you've created.
Place the lamb in a roasting tin, cover loosely with foil and place in the preheated oven. As it heats up, the butter will melt and help the rosemary and garlic to work their way into the holes and really penetrate the lamb.
After 30 minutes, remove the foil and leave to roast for a further 50-60 minutes (for medium) or until cooked to your liking.
At the end of cooking time, remove the lamb from the oven and leave it to rest in the roasting tin for 10-15 minutes, with some foil on top to keep it warm.
Make the gravy. Heat a heavy-based frying pan on the hob. When hot, pour the pan juices from the roasting tin into the pan (be careful as it may splatter). Add the red wine. Turn the heat down and leave the sauce to reduce.
To thicken the sauce, make a beurre manie by mixing together the flour and butter in a clean bowl to make a paste. Add about a teaspoon of the beurre manie to the pan and whisk well, until the sauce has the desired consistency.
To serve, slice the lamb and pour over the gravy. Garnish with a couple stalks of fresh rosemary. This would be good with potatoes dauphinoise and fresh vegetables, for a Sunday lunch.