WHAT are we to make of the extraordinary wrangling that surrounds the visit of Sir Percy Cradock to Beijing at the end of the month? Once the doyen of diplomatic mandarins and a policy adviser to then British prime minister, Lady Thatcher, it is suggested he is flying to the Chinese capital despite his government's explicit wish that he stay away. According to the Foreign Office in London, Sir Percy has been told that turning up in Beijing so soon after the third round of talks over Hongkong's constitutional future next week will be ''not helpful''. In diplo-speak, that is the equivalent of a ban. But an indignant Sir Percy, who maintains he is simply passing through on his way from a Shanghai investment company board meeting, insists his visit is entirely private. ''You cannot have people warned off the territory of China,'' he says. ''I am going privately. I shall certainly not be negotiating anything - I have no authority to do so.'' Despite such public protestations, it is surely no coincidence Sir Percy will be seeing the two key players in talks over Hongkong. He will be guest of honour at a dinner with Beijing's vice foreign minister, Mr Jiang Enzhu, and he will also meet BritishAmbassador to Beijing, Sir Robin McLaren. True, the three are old friends. And under other circumstances, such a round of socialising would probably cause little fuss. But Sir Percy is a veteran of the covert mission to Beijing. In 1991, for instance, he flew there at the behest of Prime Minister Mr John Major and successfully pulled off something of a diplomatic coup: an agreement over Hongkong's new airport which ledto the signing of the Memorandum of Understanding. Sir Percy is well aware his appearance in the city will excite comment, at the least. And a negotiator of his undoubted skill will also appreciate how one word out of place can upset months of delicate, behind-the-scenes brokering. Sir Percy has made his feelings on the way Governor Mr Chris Patten went about introducing his proposals on democracy abundantly clear. Small wonder, then, that those close to Mr Patten feel that, whatever else Sir Percy will be doing in China, he will not be supporting the reforms. Sir Percy insists that now negotiations are taking place he ''totally approves of the present British position''. But were he still a special adviser to the British Government, would he ''totally approve'' of what he is planning now? We think not. History will probably show that Hongkong owes Sir Percy a debt of gratitude for his work in helping the territory smooth its relations with China. It would be sad if such a record of service were to be spoiled by a single act of obstinacy. That is why Sir Percy, a man who has made diplomacy his life, should tell his old friends in Beijing he will have to see them some other time.