First make the sauce: mix together the honey, mustard and yoghurt. Stir in the garlic, capers, basil, mint and parsley, add the wine or vinegar and season to taste. Cover and chill.
Bring a saucepan of water to the boil, add the potatoes and a pinch of salt and cook for 10-15 minutes, adding the beans for the last 2 minutes.
Meanwhile, heat the grill to high and put a pot of water to boil. Trim the lamb of any fat. Combine the mustard powder and sugar and rub it all over the meat, then grill it for 5 minutes on each side, or until tender.
Cover the tomatoes with the boiling water, leave them for 1 minute, then skin, deseed and chop them.
When the vegetables are just tender, drain them thoroughly and mix them with the tomatoes.
Divide the potatoes and beans between four plates. Cut the lamb into thin slices and arrange them on top of the vegetables. Spoon a little sauce onto the lamb and serve the remainder on the side.