Fry the pork, onion and carrot together until the mince has coloured. Add the flour and cook for 1 minute. Add the crumbled stock cubes and add enough water so that you end up with a thick gravy. Add the tomato purée, Worcester sauce and herbs. Bring to the simmer and cook for approx 15 minutes. Add the peas and continue to cook for a further 5 minutes. Taste and add pepper if required.
Meanwhile chop the potatoes and Celeriac and place in a pan and bring to the boil until soft. Strain and mash.
Place the meat in the bottom of a dish, add the mash potato top, sprinkle. With cheese and put under a hot grill until golden brown.