Preheat oven to 170 ˚C. Heat an ovenproof pot (big enough to hold the oxtail with some room at the top) on a medium heat. Add the olive oil, leeks, celery, carrots and onions. Fry until golden brown. Add the oxtail and continue browning until all the edges of the oxtail is nicely caramelized. Add the tomatoes, muscadel, beef stock and Worcestershire and 1 litre of cold water and stir well. Bring to a boil and then place in the preheated oven for 5 hours or until the meat comes away from the bones. While the oxtail is cooking, make the polenta according to the package instructions. Pour the polenta out onto a lined baking sheet spreading to approximately 3 cm thick. Wait to cool completely. Heat a griddle pan to smoking point. Cut the polenta into triangles or wedges and place on the heated grill for 1 minute a side or until griddle marks form. Re heat in the oven with the oxtail for 10 minutes before serving with the oxtail.