Preheat the oven to 200C/400F/Gas 6.
Rub the mutton with the oil. Heat a non-reactive frying pan until hot, then fry the mutton, turning regularly, until browned on all sides.
Remove the meat from the pan (keep the pan for later) and transfer into a roasting tin. Place into the oven to roast for 25-28 minutes, or until cooked to your liking.
Remove from the oven and leave to rest, covered, in a warm place for at least ten minutes.
In the same frying pan that you fried the meat in, add the chorizo and cook until crisp.
Add the vinegar and boil to reduce the liquid by half.
Add the mustard, cayenne pepper, cooked new potatoes and a knob of butter and stir to combine.
To serve, spoon equal portions of the potato and chorizo mixture onto each plate. Carve the mutton and place slices on top of the potato and chorizo mixture. Garnish with chopped parsley.