Mix the butter, sugar, juice from half the lemon and chopped herbs together with a generous grind of salt and pepper. Rub this into the skin and cavity of the chicken. Set aside while you prepare your vegetables.
Pre-heat the BBQ with the Roast Pan in place over a medium heat for 2-3 minutes.
Add the carrots and potatoes to the pan then place the chicken on top. Add some water or stock to cover the bottom of the pan. Cover with the lid. Turn the heat down to low-medium and roast for 30 minutes.
Base the chicken with the juices regularly and add more liquid if needed.
Add the mushrooms to the pan and carry on roasting for another 40-50 minutes, or until the liquid runs clear when inserting a skewer into the thickest part of the chicken breast. Serve immediately with some steamed green vegetables.