Mix the lemon rind and juice with 4 tablespoons of the oil, the garlic and seasoning. Put the poussins into a large shallow china dish, sprinkle with half the thyme stems, then spoon the lemon mixture over the top. Cover with clingfilm and marinate in the refrigerator for 2 hours (or overnight if possible), turning once.
When ready to cook, thread 2 skewers crossways through each poussin, from the thigh to the wing. Transfer to a large roasting tin and spoon over any marinade. Put all the vegetables into a separate roasting tin with the remaining oil and thyme stems and roast in a preheated oven, 190°C (375°F), Gas Mark 5, for 40 minutes. Turn the vegetables, baste the chicken, and swop oven positions halfway through cooking until the chicken is browned and cooked when tested (see