THE most influential man in the Foreign Office is set to replace Sir Robin McLaren as British ambassador to Beijing. A British newspaper reported political director Len Appleyard has been earmarked to take up the post next year, and serve as ambassador in the run-up to 1997. The Foreign Office yesterday dismissed the report as ''speculation'', saying a decision had yet to be made. But a highly placed source in London confirmed the news. He said while the appointment had yet to be officially made, it was generally expected to go to Mr Appleyard. The 54-year-old sinologist should take over next summer, on Sir Robin's retirement after three years in the post. He is likely to remain until 1998, playing a vital role in resolving Sino-British differences in the final years of the transition, and then taking Britain's Consulate-General in Hongkong under his wing, after the change of sovereignty. A veteran old China hand, Mr Appleyard is one of Foreign Secretary Douglas Hurd's closest advisers, and also plays a key role in assisting Downing Street on foreign policy. The political director is often described as the most influential post in the Foreign Office, and Mr Appleyard was in Tokyo last week, accompanying Mr Hurd and British Prime Minister John Major at the Group of Seven summit. He previously served under Sir Percy Cradock at Britain's mission to Beijing - before it was upgraded to embassy status - during the Cultural Revolution from 1966 to 1969, when it came under attack from Red Guards. He was earlier posted to Hongkong between 1964 and 1966, and was subsequently involved in the territory's affairs as Principal Private Secretary to then-Foreign Secretary Sir Geoffrey Howe from the final stages of the Joint Declaration negotiations in 1984 until 1986. Mr Appleyard, described as a hard worker and pleasantly unstuffy, was Deputy Cabinet Secretary from 1989 to 1991, and was present in the Cabinet Room during the Irish Republican Army's mortar bomb attack on Downing Street. He has also served for three years as British ambassador to Budapest, from 1986 to 1989, and has been posted to New Delhi and Moscow, as well as doing an attachment at the Treasury in London. His post as political director is expected to be filled by Pauline Neville-Jones, 53, who is running the Cabinet Office defence and overseas secretariat. Meanwhile, Sir John Coles, who has overall responsibility for Hongkong, as under-secretary for America and Asia at the Foreign Office, has been tipped to be promoted to permanent secretary next year, the most senior post in the Foreign Office. He visitedBeijing last year as leader of the British team to the high-level airport talks.