Heat the oven to 190 C, Gas 5. Rub the chicken with the juice of 1 lemon, 3 tbsps of the olive oil and the salt. Place the chicken in a deep heavy baking dish and surround with the garlic and potatoes in a single layer.
Sprinkle the chicken with the oregano, pepper, bay leaves and the remaining olive oil. Dot with butter and add half the stock.
Roast, uncovered, for 15 minutes then reduce temperature to 180 C Gas 4 and roast until tender, basting frequently and adding more stock if needed. This will take about an hour if the chicken is whole, 40 minutes for pieces.
Transfer the chicken, potatoes and garlic to a serving dish and keep warm. Strain the pan juices into a small saucepan. Remove the fat with a spoon and add any remaining stock. If there is more than about 12 fl oz of liquid reduce it by rapid boiling.
Combine the mustard, honey and half the remaining lemon juice and stir into the sauce. Season with salt and pepper, adding more lemon juice to taste. Pour just enough sauce over the chicken and potatoes to moisten them. Sprinkle with parsley and serve the remaining sauce separately.