Preheat the oven to Gas mark 4-5, 180-190°C, 350-375°F.
Scatter the rosemary on a large tray. Unroll the joint (reserving any elasticated meat bands and coat each side with the rosemary. Leave skin side down and season.
To prepare the Christmas pudding stuffing; in a small bowl mix all the ingredients together and spread evenly over the lamb. Roll up and secure with butcher’s string or reserved elasticated meat bands. Weigh the joint and calculate the cooking time.
Place on a metal rack in a large non-stick roasting tin, drizzle with the oil and roast in a preheated oven for the preferred, calculated cooking time. Cover with foil if browning too quickly.
Remove the joint from the oven, transfer to a large plate, cover and leave to rest for 15-20 minutes.
To prepare the gravy; spoon off any excess fat from the roasting tin and discard. Place the roasting tin over a medium heat and sprinkle over the flour. Stir well with a small whisk or spoon, add a little stock and stir again, scraping the base of the pan to release any rich, sediment.
Add the remaining stock, sherry or Port and any meat juices,adjust the seasoning, if required and simmer for 8-10 minutes, stirring occasionally or until reduced to a well-flavoured gravy. Strain before serving.
Serve with seasonal vegetables and the sherry gravy.