Preheat the oven to 190°C.
Peel & finely chop the garlic the grind to a paste with a mortar and pestle. Mix together with the mustard, and oil.
Make several shallow cuts in the surface of the joint. Season with the salt & pepper then rub all over with the mustard mixture.
Put into a medium roasting tin (preferably on a rack), cover loosely with foil and roast for 50 minutes.
Place on a carving plate to rest for 10 minutes, covered with the foil to keep warm.
Serve with gravy, roast spuds and seasonal vegetables.