Cut the excess fat from the oxtail. Heat the oil in a heavy-based saucepan and brown the oxtail in batches. Return all the meat to the pan. Add the onion and garlic and cook until just softened.
Add the flour and cook, stirring, for 1 minute. Mix in half the stock and bring to the boil, stirring. Remove from the heat and refrigerate the soup for 2 hours, or until the fat can be spooned from the surface.
Add the remaining stock, 1litre (351fl oz/4cups) water, bay leaves, tomato paste, worcestershire sauce and season with salt and pepper. Bring to the boil, then reduce the heat to low and simmer, covered, for 2 hours, stirring occasionally.
Add the potato, parsnip and carrot and simmer for 10 minutes, or until tender. Remove the bay leaves before serving. Garnish with the chopped tomato and parsley.