Preheat the oven to 210°C. Cook the potatoes in a large saucepan of boiling water for 5 minutes, to blanch. Drain.
Meanwhile, place the oil and garlic in a large baking dish and heat for 5 minutes. Add the potatoes, sprinkle with salt to taste, and bake for 15 minutes, turning as they brown. Scatter over half the rosemary leaves.
Combine the oregano, chilli powder and a grinding of pepper in a small bowl. Rub the poussins with the spice mixture. Place, breast-side-up, on the potatoes and roast for 30 to 35 minutes or until cooked through. Serve the birds halved, with the lemon, remaining rosemary sprigs and the potatoes.