Put the chicken into a large bowl and pour over the buttermilk. Add the two crushed garlic cloves to the bowl, along with the rosemary and the bay leaf. Leave to marinate for 24 hours.
Preheat the oven to 140°C/gas mark 1. Remove the chicken from the marinade and place it in a roasting tray with the herbs and garlic. Add the stock to the pan and season the chicken well. Cover with a layer of foil and cook for 2 hours.
Remove the foil and continue to cook for another hour. Increase the temperature to 220°C/gas mark 7. Baste the chicken with the olive oil and cook for a further 20-25 minutes until the chicken is crisp and golden. Remove from the oven and leave it to rest.
Meanwhile, cut a row of slits into each potato, being careful not to go through the bottom. Put the potatoes cut-side up in the chicken roasting tray and brush with half the olive oil. Season well and bake at 220°C/gas mark 7 for 20-25 minutes until the potatoes start to fan out. Remove from the oven and brush with the remaining oil. Continue to bake for another 20-25 minutes until golden and tender.
Boil the green beans for 3 minutes, then drain and run under cold water. Serve with the chicken and potatoes.