Preheat the oven to 180ºC/350ºF/gas 4. Put a non-stick ovenproof frying pan on a high heat. Rub the chicken all over with ½ a tablespoon of olive oil, season with sea salt and black pepper and place skin side down in the pan. Fry for a couple of minutes until golden, then lightly squash the unpeeled garlic cloves with the heel of your hand and add to the pan. Pick in the grapes.
Turn the chicken skin side up, pour in the vermouth and transfer to the oven to roast for 40 minutes, or until the chicken is golden and tender, and the sauce is sticky and reduced. Add a splash of water to the pan and give it a gentle shimmy to pick up all the sticky bits. Pick over the tarragon, and dish up.