The third decade of The Queen's reign included a number of milestone events, causes for public and private celebration. In 1977 The Queen marked twenty-five years as Sovereign. The Silver Jubilee was met with a nationwide tour, Commonwealth visits and celebrations at every level. The Queen and The Duke of Edinburgh travelled 56,000 miles in total to mark the occasion in as many parts of the United Kingdom and Commonwealth as possible. The weddings of Princess Anne to Captain Mark Phillips (1973) and The Prince of Wales to Lady Diana Spencer (1981) were also cause for widespread public rejoicing. The Queen and Duke of Edinburgh themselves celebrated 25 years of marriage in 1972 with a service of thanksgiving at Westminster Abbey. In 1977 they gained their first grandchild, with the birth of a son to Princess Anne. The decade had its share of personal grief. The Queen's uncle, The Duke of Windsor and former King Edward VIII, died in Paris on 28 May 1972 at the age of 77. The Queen's 30-year-old cousin, Prince William of Gloucester, was killed in 1972 in a flying accident, as a result of which The Queen cancelled her visit to the Munich Olympics. In 1979 Earl Mountbatten of Burma was assassinated by an IRA bomb blast while the Earl was fishing in Co Sligo, Eire. Cousin to The Queen and uncle to The Duke of Edinburgh, Earl Mountbatten was First Sea Lord and Chief of the Defence Staff; he had also been the last Viceroy of India before independence. The Queen's political duties continued as usual. In 1974, The Queen had to fly back from an Australian tour when a general election was suddenly called - it is the only time she has done so. Following the general election in February, no one party had a clear majority in the House of Commons. The question arose as to whether The Queen should endorse a coalition between the Conservative and the Liberal parties without first giving Labour, the largest party, a chance to form a government. The Tories and Liberals were in the end unable to arrange a coalition, and Harold Wilson returned to power as prime minister. Later in the decade, The Queen received the first female prime minister at Buckingham Palace, when Margaret Thatcher replaced James Callaghan as leader in 1979. On the international stage, too, there were several 'firsts'. The Queen paid an historic visit to communist Yugoslavia in 1972, travelled to Japan as the guest of Emperor Hirohito in 1975, was the first British Sovereign to travel to the Middle East in 1979, and visited Pope John Paul II in the Vatican in 1980. The Commonwealth went from strength to strength. The Queen, Head of the Commonwealth, visited Ottawa at the time of the Commonwealth Heads of Government Meeting being held there. Since then The Queen has been present in the host country for every CHOGM, in Britain or abroad, to demonstrate her commitment to the organisation and its principles.