Preheat oven to 190°C, fan 170°C, gas 5. In a food processor, whizz together the olives, sundried tomato paste, lemon juice and zest, chilli oil and parsley for the tapenade. Set aside.
To prepare the chickens, use your fingertips to gently loosen the skin covering the chicken breasts. Spread the tapenade under the skins, being careful not to tear them. Set aside while you prepare the vegetables.
Gather 3 leek halves together in a bundle and tie tightly with a piece of butcher's string. Repeat with the remaining leeks and transfer to a large roasting tin (big enough for two chickens) along with the halved garlic bulbs and onion wedges.
Place the chickens on top, and pour over the wine and oil. Tightly cover the breast of each chicken with a piece of foil and transfer to the oven for 1 hour 30 minutes.
Meanwhile, combine the potatoes, chopped garlic and oil in a roasting tin, season well and transfer to the oven for the final 45 minutes.
Remove the chicken from the oven and pierce the thickest part of the thigh with a skewer. If the juices run clear the chicken is cooked, if they are pink return to the oven for further cooking. Allow the chicken to rest for 10 minutes before carving.
Remove the potato wedges from the oven when they are crisp and golden. Toss with the basil and serve immediately.
Cook's tip: to remove the leg joint, cut between the body and the drumstick and lever away from the body. Cut around to remove completely. Finally, cut through the joint between the leg and thigh.