Heat the oil in a large lidded saucepan or a high-sided non-stick frypan, add the onions and lamb. Stir over medium heat until the lamb is browned and sealed, then add the carrots, celery and leeks. Stir and cook for a further 5-7 mins. Pour in the chopped tomatoes and the paste mixed with water then mix in the sultanas. Season with salt and pepper and stir in minced chili. Stir well, cover with the lid and simmer on a low heat for 25-30 minutes, stirring occasionally. .
Bring two cups of water to boil in a lidded saucepan with 1-2 tablespoons of oil and a little salt. Immediately remove from heat, stir and pour in the couscous. Cover and allow to stand for 2-3 minutes while the water is absorbed. Add 1-2 teaspoons of butter and fold through the grains with a fork or spatula. The grains should hopefully separate nicely. Reheat gently if necessary or serve immediately as a bed under your yummy lamb stew.