Zest the whole lemon, cut in two and squeeze the juice from one half. Place the juice and zest into a large shallow dish then snip the rosemary into the dish too. Add 3 tablespoons of the olive oil, the cider vinegar, salt and freshly ground black pepper. Place the noisettes into the dish and make sure they are well soaked in the marinade. Cover and refrigerate for 2-3 hours.
Heat the remaining oil in a frying pan, remove the lamb from the marinade (reserve for later) and sear the lamb for 2-3 minutes, turning once until brown on both sides. Transfer to a hot roasting tin, add a little of the marinade mixture and cook in a preheated moderately hot oven (Mk 6 - 400ºF - 200ºC) for a further 6 -8 minutes according to how pink you like your lamb. Serve hot with sauté potatoes