Boil the potatoes in lots of salted water until almost cooked all the way through. Drain and leave to cool a bit. Strip the rosemary leaves and mix with the oil, garlic and seasoning in a roasting tray.
Tip in the potatoes, crush them a little with a masher or a fork and toss with the oil mixture. Roast for 35 mins in the oven underneath the chicken until they are crunchy and you can smell the rosemary and the garlic just on the edge of burning.