Remove the chicken from the fridge at least 30 minutes before cooking to allow to come to room temperature. Heat oven to 190°C.
Whilst waiting for the chicken and oven to reach correct temperature, Remove the outer leaf of the leak trim off the papery, dark green tops and chop into approx 2 cm thick logs before washing, thoroughly under cold water. Cut the peppers into thick strips and discard the seeds and top. Lay the sliced peppers and leeks into the base of a roasting pan.
To make the Marmite butter. Strip the leaves off the stalks of the thyme and add to a small bowl along with the butter and Marmite and mix until smooth and fully combined.
Place the chicken on top of the vegetables in the roasting pan. Lift the skin on the breast near the cavity and add a blob of Marmite butter into each. Then massage the chicken skin to spread the butter. Using the remaining butter smother the rest of the chicken.
Place in the middle of the oven and roast for 1hr and 20 minutes. To check, pierce the thigh with a skewer and the juices should run clear.
Carefully remove the chicken from the oven and transfer to a serving plate along with the caramelised vegetables. Allow to rest for 15 - 20 minutes before carving.