Heat the oil in a large frying pan. Fry the thighs for 8-10 mins, skin side only, until golden brown, then remove to a casserole dish. Fry the drumsticks for about 5 minutes on each side and tip in with the thighs.
Pour over the stock in the casserole dish. The stock should cover the chicken, add more water if needed. Bring stock to the boil and cover loosely with a lid. Cook just below the point of simmering for 30-35 mins until chicken is cooked.
While the chicken is simmering, drain oil from the pan. Heat the butter in the pan and add the onion. Gently fry the onion for 5 mins until softened.Increase heat, tip in the mushrooms and then fry for 3 mins until they soften. Pour in the white wine, turn the heat up and boil rapidly for 6-8 minutes until the sauce has reduced. Turn off the heat and leave until chicken has cooked.
Once chicken legs are cooked, strain stock into pan with the onion, mushrooms and white wine, bring back to the boil and reduce again by two-thirds until the sauce has thickened. Pour in the double cream, bring it to the boil, add a little salt and pepper if desired and pour it over the chicken. Heat the chicken in the sauce for about 2-3 minutes, remove from heat and allow to stand for a few minutes before serving.