Preheat the oven to 200C/400F/Gas 6. Untruss the chicken, sit it in a roasting tin and put the lemon halves and two of the rosemary sprigs into the chicken’s cavity.
Wash and trim the leeks, cut each into three logs, then slice lengthways and add to the tin. Now remove the core and seeds of the peppers and slice them into strips, following their natural curves and ridges, and add these to the tin.
Tumble in the olives, and now pour the olive oil, mostly over the vegetables but a little over the chicken, too. Add the remaining rosemary sprigs to the vegetables, along with some sea salt flakes and freshly ground black pepper, to taste. Using a couple of spoons or spatulas, gently toss the vegetables about to help coat them with the oil and make sure everything’s well mixed up.
Sprinkle some sea salt flakes over the chicken and put it in the oven for about 1-1¼ hours, by which time the chicken should be cooked through, and its juices running clear when you cut into the flesh with a small sharp knife at the thickest part of the thigh joint. The vegetables should be tender by now, too, and some of the leeks will be a scorched light-brown in parts.
Remove the chicken to a carving board and, while it rests (for about 10 minutes) pop the pan of veg back in the oven, switching the oven off as you do so.
Cut the chicken up chunkily, transferring the pieces to a large warmed platter. Now take the pan back out of the oven and, with a slotted spoon or spatula, remove the vegetables to the large platter and when all is arranged to your aesthetic delight, pour over it all the bronze, highly-flavoured juices that have collected in the pan.