Approximately how many tv-viewers world-wide tuned in for Lady Di's funeral?

Explanation

Diana, Princess of Wales was the first wife of Charles, Prince of Wales, whom she married on 29 July 1981 and divorced on 28 August 1996. She died in a car crash in Paris along with her companion Dodi Fayed on 31 August 1999. It was estimated that a total of 2.5 billion people watched the princess' funeral, a testament to her global celebrity. The wedding, which was held at St. Paul's Cathedral in 1981, some 18 years earlier, attracted a global audience of over 750 million people - then a record.