Gregg Wallace is following production of a seasonal favourite, the Yorkshire pudding. Inside the factory in Hull, they produce five hundred million frozen Yorkies every year. Although the factory makes them by the tonne, the ingredients are as simple as when you make them at home – flour, eggs, milk and salt. But the processes are entirely industrial, with high tech mixers, a 53 metre-long conveyor oven and huge freezers.Gregg learns that the key to getting the perfect Yorkshire pudding shape is to trap air in the batter while mixing, and to make sure the oil added to the cooking pan forms a ‘halo'. Elsewhere in the episode, Cherry Healey visits one of the country's largest wheat testing and storage depots where each silo contains enough wheat to make flour for 416 million Yorkshire puddings. And if you prefer beef to your Christmas turkey, she teams up with food scientist, Barbara Bray to learn how to cook the perfect gravy for a Sunday roast. And historian Ruth Goodman bites into the history of the roast dinner; and helps a chef master the art of washing up Tudor-style.