Make deep diagonal slashes in the lamb as if you were crisscrossing a baked ham. Push 3/4 of the chopped garlic into the cuts and sprinkle with the herbs. Season with salt and pepper and dribble olive oil over the whole leg.
Layer the potatoes in a roasting tin, interspersing the layer with salt, pepper, dots of butter and the remaining garlic. Put the meat on a roasting rack over the potatoes.
Set the tin on the bottom shelf of the preheated oven and roast for 90 minutes if you like the meat pink . If you like it more well done reduce the heat and cook for a further 30 minutes.
Remove the meat from the potatoes and cover with foil to allow it to rest for 10 minutes. During this time you can brown and crisp the potatoes on a high shelf in the oven.