The red double-decker bus is a global icon. They carry millions of passengers every day across the capital and are as synonymous with London as Tower Bridge and Buckingham Palace. Now, Gregg Wallace has special access to a factory in Scarborough, Yorkshire where they build this famous people mover. But the bus that Gregg is helping to produce is a little bit special, because it's fully electric.Gregg helps the factory across all stages of the bus's construction, including operating a crane to lower the bus's steps in place, adding the anti-slip lino, riveting and gluing the walls and wiring the electrics - before taking on the nerve-wracking task of driving the finished bus out of the factory.Meanwhile, Cherry Healey visits a bus windscreen factory where she gets to grips with the construction of tough laminated heated windscreens. And in the main bus factory, she helps to give the bus its bright red coat of paint. She also visits an offshore windfarm to learn how turbines convert wind into watts that could one day power the electric buses.Historian Ruth Goodman learns about London's earliest double deckers and the vital role they played in the First World War.