Heat oven to 200C/180C fan/gas 6. Pick the tops off all the rosemary sprigs and set them aside, then stuff the remaining stalks into the chicken cavity and sit it on top of a large roasting tin.
Drizzle with 2 tbsp of the oil and season well. Cover with foil and roast in the oven for 1 hr 20 mins, removing the foil for the last 20 mins.
Just after the chicken goes in, put the vegetables in a second roasting tin along with the the rosemary tops.
Drizzle with the remaining oil, season and put in the oven underneath the chicken for 1 hr-1 hr 10 mins or until soft and lightly browned.
Check the chicken is cooked by piercing the thickest part of the thigh with a skewer – the juices should run clear; if not, put it back in the oven and test again after another 10 mins.
Carefully remove the chicken from the roasting tin and rest it on a board 15-20 mins.
As the chicken rests, keep the vegetables warm and make the gravy. Put the roasting tin on a hob over a low heat, add the gluten free flour, mix well until it forms a paste, then pour in the stock, stirring constantly until thickened.
Simmer for 5 mins, then season to taste. Strain into a gravy boat or heatproof jug and serve with the chicken and vegetables.