Separate and peel the garlic cloves, and crush them with the salt in a pestle and mortar. You need enough salt to give the mixture some grit. Mash the garlic into the salt. Add the rosemary and thyme, then mix with enough olive oil to make a thick paste.
Rub the garlic mixture over the whole surface of the lamb. Place the lamb into a deep sided roasting tin.
Add 250ml/9fl oz of water to the roasting tin. Baste the meat with the liquid then cover the lamb with foil. Place in the oven and roast for 3-3½ hours, until cooked to your liking.
When cooked, pour off any excess oil from the top of the liquid, leaving the cloudy, herby sediment in place. This is the gravy. Cover the pan with a lid and rest for 15 minutes.
Cut up the potatoes and cook in boiling, salted water. When soft enough to mash, drain in a colander then mash with the butter and grated parmesan. Season well with salt and pepper.
Carve the lamb into thick chunks and serve with the mashed potatoes. Pour over the pan juices.