Fans of Countdown are to see a big change to proceedings with Rachel Riley not in her usual role. Instead, a YouTuber has taken over the reins. Dr Tom Crawford, who is known for his channel in which he teaches sum in his underwear, is standing in for the mathematician who is currently working on another project. He is a pal of the star and is said to have been shown the ropes by her ahead of his own stint. The 35-year-old has over 200,000 subscribers on his own channel, in which he describes