Heat the oil in a deep frying pan over a moderate heat and fry the chicken for 5 minutes until golden brown all over. Remove the chicken to a plate.
Pour the vinegar into the pan and cook until reduced by half, scraping the bottom of the pan continuously with a wooden spatula. Add the chopped tomatoes, stock, thyme, garlic, paprika or cayenne and sugar to the pan. Bring to the boil, then reduce the heat to low and return the chicken to the pan. Cover and cook gently for 30 minutes until the chicken is tender.
Remove the chicken and arrange on a heated serving dish and keep warm. Discard the thyme from the sauce. Tip the sauce into a blender or processor and whizz until smooth. Pass the sauce through a sieve if you prefer a smoother texture. Season to taste, then pour over the chicken. Garnish with sprigs of fresh thyme.