Preheat the oven to 200°C. Line a baking tray with nonstick baking paper.
Rub the chicken thighs with the groundnut oil and season well. Roast for 25 minutes in the oven. Cook the potatoes in a large saucepan of boiling, salted water for 20 minutes until tender. Meanwhile, prepare the sauce by bringing the vinegar to the boil in a small saucepan.
Add the sugar and swirl to dissolved it in the hot vinegar. Add the raspberries and cook the sauce for 10-15 minutes over a low heat until it has reduced by half and had a thin syrup consistency. Press the sauce through a sieve in to a small, clean saucepan and set to one side.
Drain the potatoes when they are tender and leave them to cool for a few minutes. Mash with the butter and seasoning until smooth. Add the chopped herbs and mix well to incorporate them. Cover and leave to one side.
Preheat the grill to hot.
Remove the chicken after 25 minutes in the oven. Pierce the skin to check the meat is cooked through with no pink showing and the juicing running clear. Place the chicken thighs under the grill for a few minutes to help the skin crisp. Reheat the sauce and mash at this point. Spoon the mash onto serving plates.
Remove the chicken thighs and sit next to the mash. Drizzle the sauce around the chicken, then garnish the chicken with some flaked sea salt and ground black pepper. Garnish the plate with a sprig of chervil before serving.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.