Stir together the marinade ingredients and season with ½ tsp ground black pepper and 1½ tsp salt. Slash the lamb several times on both sides, then massage the marinade all over it. Seal into a large food bag or non-metallic container and chill overnight (or at least for a few hrs). Let the lamb sit at room temperature for 1 hr before roasting.
Heat oven to 200C. Put the lamb into a foil-lined roasting tin and roast for 20 mins. Turn the oven down to 190C and roast for 1 hr 20 mins for meat that’s pink near the bone. Cover loosely with foil halfway through cooking or once the marinade has charred and the meat looks golden.
Leave the lamb to rest for 20 mins before carving.