Preheat the oven to 200°C (400°F/Gas 6). Wipe the chicken with paper towels and put breast-side-down in a large roasting tin. Put a little salt, a garlic clove, 3 of the thyme sprigs and 1 of the bay leaves in the cavity of the chicken. Scatter the potatoes and remaining garlic around and pour the lemon juice over the top. Rub the skin of the chicken with some of the butter and dot the rest over the potatoes. Bury the rest of the thyme sprigs under the potatoes, then sprinkle salt over the potatoes and the chicken. Pour 250 ml(9 fl oz/1 cup) of water around the edge of the dish.
Roast for about 1 hour or until the chicken is nicely golden, then turn it over and shuffle the potatoes around. Spoon the pan juices over the top of the chicken and potatoes and scatter some salt over the new top of the chicken. Roast for about 30 minutes, shuffling the potatoes around again halfway through without breaking them up too much, or until the chicken is deep golden and crispy and its juices run clear. Transfer the chicken to a generous serving platter with a bit of a raised edge and arrange the potatoes around the chicken. Keep warm.
Put the roasting tin of cooking juices over medium heat on the stovetop. Using a wooden spoon, scrape up all the golden bits from the sides and bottom of the tin. If there isn’t much liquid, add 3–4 tablespoons of water. Bring to the boil and cook until slightly thickened. Stir the cream through and let it all bubble up, whisking so it all comes together as one. Pour over the chicken on the platter and serve immediately.