Place the chicken, carrots, celery, potatoes and thyme into a large, deep pan and pour in 3 litres of water or enough to cover the chicken.
Simmer on a medium heat for 1½ hours or until the chicken is cooked through.
Remove the chicken from the pan and strain the broth. Save the veg for later. Put the broth back on a high heat and allow to reduce for 15 minutes until there’s about 2 litres left.
Meanwhile, tear the cooked chicken into long chunks. Once the broth has reduced, throw the vegetables back in the pan with the peas, leeks and chicken. Simmer for a further 5 minutes, then remove the sprigs of thyme.
Serve in warm bowls sprinkled with chopped parsley and the celery leaves.