Place the joint in a roasting tin with the fatty side up. In a small bowl, mix together the garlic, olive oil, salt, pepper and thyme. Spread the mixture over the fatty layer of the joint, and let the joint sit out until it is at room temperature, no longer than 1 hour.
Preheat the oven to 260 C / Gas 9.
Roast the joint for 20 minutes in the preheated oven, then reduce the temperature to 160 C / Gas 3, and continue roasting for an additional 60 to 75 minutes. The internal temperature of the roast should be at 55 degrees C for medium rare.
Allow the roast to rest for 10 or 15 minutes before carving so the meat can retain its juices.