Heat the oven (350F/180C/Gas Mark 4-5). Make the stuffing as directed on the packet, adding lemon juice to the hot water before mixing the stuffing to give a tangy flavour.
Spread the stuffing over the lamb, and roll it up carefully, not too tightly. Tie it up in 2 or 3 places with the string, or secure in a roll with cocktail sticks. Lightly rub the outside of the meat with the oil, and either wrap the meat in the foil and place it in a roasting tin, or put the meat in a greased roasting tin and cover the tin with the foil.
Cook in the oven to 1 1/2 to 2 hours, according to the size of the joint (a bigger joint will take longer) unwrapping or removing the foil for last half hour of the cooking time to brown the meat.