The Christmas Number One is a British tradition: but it's one that's having to go through some changes -- because not many people buy music any more. Here's how the charts are calculated these days, and why listening to "All I Want For Christmas" on repeat isn't going to change who wins. Thanks to Martin Talbot and all the team from the Official Charts Company, who agreed to an interview on less than two hours' notice for this video! You can visit them here: http://www.officialcharts.com/ or on Twitter at @officialcharts