1. Place all the ingredients for the marinade in a bowl and mix together.

2. Place the lamb in a glass dish and spread the marinade all over it. Cover the dish and marinade the lamb in the fridge for 2 days, turning once or twice during that time.

3. To cook, drain the lamb and reserve the marinade. Heat the grill or barbecue to hot.

4. Wipe the lamb dry and place under the heat (or over it) and cook for 10 minutes, basting with some of the marinade. Turn the lamb over and cook for a further 10 minutes.

5. Brush the cooked lamb with any remaining marinade and leave for 5 minutes before slicing.

6. The 10 minute cooking time is based on a leg of lamb weighing 3.2kgs. Cooking times will vary according to the size of the piece of lamb and the preferred degree of cooking.

Sauce

7. Bring the tomatoes and wine to the boil. Lower the heat and simmer for about 20 minutes, or until the mixture is thick.

8. Press the mixture, plus 4 tbsp of the reserved marinade through a fine sieve into a heatproof bowl.

9. Beat the mustard into the mixture and then set the bowl over a pan of simmering water. Beat in the butter a little at a time, and when fully incorporated. Serve with lamb.