Place chicken breasts in a shallow dish. Pour 1 cup maple syrup over chicken. Peel head of garlic and crush cloves under the flat-side of a knife and add all but 2 cloves to chicken. Add 1/3 cup malt vinegar and toss chicken to coat. Let marinate from 1 to 6 hours.
Preheat oven to 190°Cand grease a roasting pan (or line a baking tray with parchment paper). Place chicken breasts in roasting pan, shaking off excess syrup, and season. Roast uncovered for about 25 minutes, until an internal temperature of 82°C is reached. Let chicken rest 10 minutes.
To serve, heat remaining 1/2 cup maple syrup, 3 tbsp malt vinegar and 2 cloves of garlic. Simmer for 3 minutes, then remove garlic and keep warm. Slice chicken breasts into 3 pieces, on an angle and plate. Spoon warm syrup over and serve.