For the chicken, heat the vegetable oil in a heavy-bottomed saucepan. The oil is ready when a breadcrumb dipped in will sizzle gently in it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Coat the chicken in the flour, dip in the egg, then coat in couscous. Deep fry the chicken for 5 minutes until golden and thoroughly cooked through. Remove with a slotted spoon and drain on kitchen towels.
To make the sauce, heat the oil in a frying pan. Add the leek and chilli and fry for 1 minute. Add the chopped tomato, tomato purÃ©e, ketchup, vinegar, sugar and half the coriander. Cook for 2 minutes.
Mix the cooked couscous with the tomato sauce and season well with salt and freshly ground black pepper.
To serve, spoon the spicy couscous into the centre of a warm plate, place the chicken on top and sprinkle with the remaining chopped coriander.