Pour the roasting juices from the tray into a clear jug and allow them to settle. Pour the gravy back into the pan over a medium heat and discard the fats.
Pour 850ml of chicken stock into the pan, using a wooden spoon to scrape up the pieces from the bottom. Bring to the boil.
Mix together 30g of room temperature butter with 60g of flour and whisk some into the gravy to thicken it.
Season with salt and pepper, strain and serve.