Rub the chicken inside and out with salt and freshly ground black pepper. Cover and sit in the fridge for an hour plus. Remove from the fridge half an hour before cooking.
Preheat your oven to 190ºC/375ºF/gas 5.
Bring a pan of salted water to the boil.
Put the potatoes into the water with the whole lemon and the garlic cloves. Cook for 12 minutes. Drain and allow to steam dry for 1 minute. Remove the lemon and garlic. Toss the potatoes in the pan while still hot so their outsides get fluffy.
While the lemon is still hot, stab it about all over with a knife tip.
Rub the chicken all over with olive oil. Push the garlic cloves, the whole lemon and the thyme into the cavity, then put the chicken into a roasting tray and cook in the preheated oven for around 45 minutes.
Remove the chicken to a plate. Some fat should have cooked out of it into the roasting tray, so toss the potatoes into this with the rosemary leaves. Shake the tray around, then make a gap in the centre of the potatoes and put the chicken back in.
Cook for a further 45 minutes, or until the chicken is cooked and the potatoes are nice and golden.