For the lamb, place all of the lamb ingredients except for the lamb and the butter into a bowl and mix well. Rub the marinade onto the lamb to coat all over, then place into a large dish and set aside to marinate for two hours.
Preheat the oven to 200C/400F/Gas 6.
Shake off the excess marinade from the lamb and place into a roasting tin. Transfer to the oven and cook for ten minutes, then baste with the remaining marinade. Return to the oven for a further five minutes, or until the lamb is cooked to your liking. Drizzle over the melted butter and set aside in a warm place.
For the mint chutney, bring a small pan of water to the boil. Add the mint leaves and return to the boil.
Drain the mint leaves and place into a bowlful of iced water to refresh them, then drain again.
Place the mint leaves into a food processor with the remaining mint chutney ingredients and blend until smooth.
For the mango salad, place all the mango salad ingredients into a bowl and stir.
To serve, carve the lamb into portions and place onto serving plates with a spoonful of the mango salad and a dollop of mint chutney.