1. Preheat the oven to 190C/375F/gas mark 5. Place the lamb in a roasting tin. Mix the butter, honey and rosemary together and spread all over the lamb. Pour three-quarters of the cider into the tin and loosely cover the lamb with a large sheet of foil.
2. Cook the lamb towards the centre of the oven, allowing 55 minutes per 1 kilo, plus 35 minutes. Remove the foil three-quarters of the way through and continue roasting the lamb, basting frequently with juices from the tin.
3. Lift the joint out of the tin and keep warm. Strain the juices from the tin, removing any excess fat, then pour the remaining cider into the tin to deglaze it, stirring as you do so.
4. Bring the cider to the boil in the tin, then return the cooking juices. If you like, thicken it with a little arrowroot, allowing 1 tsp arrowroot for each 300ml liquid. Mix the arrowroot with a little cold water and stir in carefully, then simmer for 1-2 minutes to create a smooth sauce. Season to taste.
5. Carve the lamb and serve with the sauce and a mixed mash of carrots, parsnips, turnips and potatoes.