Preheat the oven to 200ºC/fan 180ºC/gas mark 6. Place the chicken in a large roasting tin. Scatter over the crushed garlic and mixed herbs. Drizzle with half the oil and toss all together.
Place the sliced potatoes in another large roasting tin. Drizzle with the remaining oil, season and toss to coat. Place the two roasting tins in the oven, with the chicken above the potatoes, and roast for 30 minutes, turning halfway through cooking.
Meanwhile, stir-fry the peppers for 5 minutes in a large frying pan over a medium-high heat, then drain off any excess water.
Tip the fried peppers into the tray of potatoes and add the olives. Transfer the chicken into the potato tin and toss everything together. Return to the oven for a further 15 minutes, until the chicken is golden and cooked through. Transfer to a warmed dish and serve.