Roast the chicken carcasses and bones in a baking tray lined with greaseproof paper, and pop in the oven at 180 degrees until cooked and golden, about 45 minutes.
While the chickens are roasting pop all your vegetables in a thick oven proof dish.
Once the chickens are cooked, add them and all their juices into the oven dish.
Cover with boiling water and bring to a boil on the stove. Once boiling, skim off any foam that has formed then pop the dish with the lid on in the oven for at least 12 hours at 110 degrees.
Remove from the oven and run through a sieve collecting the liquid, and disregarding the bits you remove. And there you have your broth. You can freeze this, or pop it in the fridge, or drink it straight away.