ONE of the Foreign Office's most experienced officials is set to be posted to Hongkong later this year as Governor Mr Chris Patten's top aide on relations with Beijing. Sinologist Mr Robert Peirce, 37, is awaiting official confirmation to the post of Political Adviser, now held by Mr William Ehrman. But it could yet be vetoed by Mr Patten. Speaking from New York, where he is serving at the British mission to the United Nations, Mr Peirce said he hoped to arrive in the autumn, and had already discussed the move with Mr Ehrman. ''I'm confidently expecting that to happen, although I've still not been officially informed by the Foreign Office personnel department,'' he said. The move will complete the line-up of Foreign Office diplomats in the Hongkong Government's most sensitive office, which handles relations with China. Mr Peirce is one of Britain's most experienced officials on the territory's affairs, having spent most of the past 13 years working in Hongkong-related posts. ''I've had a wide range of jobs dealing either partly or totally with Hongkong so it feels pretty much like home,'' he said. He is close to leading old China hands in the Foreign Office, such as Ambassador to Beijing Sir Robin McLaren, former Governor Lord Wilson, and former Prime Ministerial Adviser Sir Percy Cradock. But the new Political Adviser said he was not a sinologist. ''I'm not one of those dry academic sinophiles and it is Hongkong I consider to be my specialisation.'' Mr Peirce said he had no misgivings about taking over a post at the centre of political controversy and other issues, with the Political Adviser being called to Government House on an almost daily basis. ''I never expected it to be a doddle. There's going to be a lot of work to do, but I never expected otherwise,'' he said. ''I'm totally dedicated to Hongkong and, frankly, I thought this was a job I absolutely have to do.'' Mr Ehrman is expected to leave in August after a standard four years in the post, while Mr Peirce will finish a three-year term in July as a counsellor in New York, handling European and Asian affairs at the UN Security Council. He served in Hongkong as Deputy Political Adviser for two years from 1986, before handing over to Mr Stephen Bradley in 1988. He returned to Britain as Assistant Private Secretary to then Foreign Secretary Lord Howe. The Political Advisers Office handlesHongkong relations with foreign countries, and is invariably staffed by Mandarin-speaking diplomats, since most of its work concerns relations with the mainland.