Peel and crush (mince) the garlic and mix with the thyme. With a sharp knife, make slits in the lamb and stuff with the thyme and garlic paste. Place the lamb in a roasting tin and drizzle over the olive oil, then season with salt and freshly ground black pepper to taste. Roast in a preheated moderately hot oven (Mk 5 - 375ºF - 190ºC) for 30 minutes per lb (per 450g) making sure you baste the joint at least 3 times while it is cooking. When it is cooked as you like it, remove it to a carving board and keep it in a warm place to rest for 15-20 minutes.
Mash the anchovies and squeeze the juice from the orange. Make a gravy by using the lamb essence that remains in the roasting tin and adding the anchovies, orange juice and crême fraiche. Stir whilst heating, but do not allow to boil.