Heat your slow cooker. Drizzle the oil in a wide frying pan over a high heat. Season the pork, then add to the hot pan. Avoid overcrowding the meat – you may want to do this in batches. Cook until deep brown all over, then transfer to the slow cooker. Add the onion and leeks to the frying pan and cook for a few mins, until they soften. Add a splash of water and scrape any tasty bits from the bottom, then tip everything into the slow cooker. Add the carrot, herbs, stock cube, mustard and vinegar, season, then add enough water to just cover the ingredients. Stir, then set your slow cooker on low for 6-8 hrs, or high for 5-6 hrs.