Preheat your oven to 160°C/140°C fan/gas mark 3. Lightly oil the inside of a medium baking tray. Roughly chop the leaves of half the rosemary and mix with 1 tbsp olive oil, the garlic, and a good amount of seasoning. Rub over the entire chicken and then place in the roasting tin.
Add the lemon, onion, tarragon and remaining rosemary to the cavity of the chicken. Pour the wine and half the stock into the tray with the chicken. Cover with foil and transfer to the oven. Cook for 1 hour.
Meanwhile, wash and dry the grapes, keeping them on small stalks. Toss them in the remaining oil and season.
After 1 hour, remove the chicken from the oven. Turn the heat up to 200°C/180°C fan/gas mark 6. Remove the foil and arrange the grapes around the chicken, then roast for a further 45-50 minutes, basting halfway through.
Once done, the chicken should be crisp and golden on the outside and the juice should run clear from the thigh. Remove the chicken from the tray and transfer to a plate. Cover and rest for 15 minutes. Strain all the juices from the tray into a saucepan and transfer the grapes in the tray back into the oven. Roast for a further 5 minutes until sticky and begin to caramalise.
Add the remaining stock to the saucepan and bring to the boil for 10 minutes or until the juices have reduced and thickened. Serve the chicken with the sticky, sweet grapes and jus.