Toss the potatoes in a roasting tin with all the other ingredients except the lamb. Trim any scrappy bits of fat off the lamb.
Set the lamb on top of the potatoes, season and drizzle with a little olive oil. Cook in an oven preheated to 200 C, 180 Fan, Gas 6 for 1 1/4 hours.
Leave to rest, covered with foil and insulated with tea towels or similar for 15 minutes. Serve on a platter surrounded by the potatoes and roast tomatoes.