Peel half the garlic cloves and lightly crush them in a pestle and mortar with some sea salt. Mix in the thyme leaves and paprika. Gradually add the oil, grinding until you have a thick paste.
Melt the butter in a small pan and stir into the spice paste.
Put the lamb into a deep sided roasting tin and rub it all over with the spice paste. Season with sea salt and black pepper then add the whole sprigs of thyme and the remaining garlic cloves still in their skins. Place the lemon halves beside the lamb.
Put into the oven and cook, uncovered, for 35 minutes. Remove from the oven, pour in the water and use a ladle to baste the lamb with the water and cooking juices.
Cover the lamb with foil and return to the oven to cook for a further 3 hours, basting every hour. For the last 15 minutes of cooking time remove the foil. If the juices are evaporating too quickly add a little more water.
Remove from the oven, cover in foil and allow to rest for at least 15 minutes before slicing.