Peel 3 of the garlic cloves and cut each into three. With the help of a small, pointed knife, pierce the flesh of the lamb to a depth of about 1" (2.5cm) and slide in a few grains of coarse sea salt. Add a piece of garlic and reclose the slit with your fingers. Repeat this nine times around the joint.
Remove the leaves from the thyme (you can use 3 teaspoons of dried thyme if necessary, but fresh thyme has more taste). Grind the bay leaves and add both the bays and the thyme to 6oz (175g) of the butter in a bowl. Mix well.
Season the lamb well with salt and freshly ground black pepper then coat it in a layer of perfumed butter. Wrap in aluminium foil and store in the fridge overnight.
The following day, take the lamb from the fridge at least one hour before cooking. Peel and thinly slice the potatoes (if they are too thick, they won't have time to cook). Rinse the potatoes in cold water to remove excess starch.
Peel and crush (mince) the remaining three cloves of garlic. Generously butter a large earthenware or other oven-proof dish, add a layer of potatoes, sprinkle with salt, freshly ground black pepper, crushed garlic and small pieces of butter. Repeat this until all the potatoes are used up, finishing with butter, garlic, salt and pepper.
Preheat the oven to moderate (Mk 3 - 325ºF - 170ºC). Place the potatoes right at the bottom of the oven. Slide an open oven rack above and put the lamb directly on it, above the potatoes, so that the juices "weep" onto the potatoes during cooking. Cook for 1½ hours (for medium rare) or 1¾ hours (for medium well).
Remove the lamb from the oven, cover and allow to stand for 15 minutes before serving. Check that the potatoes are cooked (try one!!) and allow a little longer if necessary; otherwise, simply keep them warm in the oven. Carve the lamb and arrange the slices on top of the potatoes. Pour the juices from slicing over the dish... Wow!