THE career options available to the retired Foreign Office mandarin ought to be enough for the most demanding diplomat. Clutching his newly minted KCMG he may, if he wishes, retire on a generous pension to cultivate his garden, while dabbling, perhaps, in the affairs of the local District Council as a sideline. If he hankers for a more active existence he may consider a variety of alternatives. Respectable charities will happily recruit him to add a touch of class to their governing bodies. Businesses will pay a fat fee to put his ambassadorial gloss on the board (though offers of this kind need to be considered with some care). If academically inclined he can look with considerable confidence to a chair at some university taking an interest in the site of his last posting. If he has the right connections (and a deplorable number of former ambassadors do) then the headship of an Oxbridge college is a possibility. If he has literary gifts, and has enjoyed postings to interesting countries which produced stirring events, he can write his memoirs. These are not negligible choices. Men of great experience and sagacity have constructed interesting second careers out of them. So what has got into Sir Percy Cradock? Sir Percy was, no doubt, a distinguished ambassador to Beijing. Standards are generous in these matters. Anyone who was not caught absconding with the embassy silverware or selling the secret files to the local army qualifies for the ''distinguished'' epithet once he retires. Far be it from me to disturb the tradition. He was already stretching his shelf life when he reappeared as a personal adviser to Mrs Margaret Thatcher. Courtiers of this kind are seldom much loved by those outside the inner circle. And Sir Percy was surely well past his ''sell by'' date when he continued to offer advice to all and sundry after the Blessed Margaret and her entourage had been expelled from Number Ten. And still the man will not shut up. I mean write your memoirs by all means - but letters to the Foreign Affairs Committee? IT seems to me that Sir Percy's current verbosity - whatever you think of the contents - is a danger to two useful conventions. Thefirst one is that advice, however well-prepared, is still advice. The person to whom it is offered retains the right of, and the responsibility for, the final decision. Advisees do not, in the polite tradition, blame their advice if the result of following it is catastrophic. And the advisers do not bleat, at least in public, if their advice is spurned. This is not just one of those fusty conventions which litter the English constitution. It recognises the enduring reality that making suggestions, however practical, is not quite the same thing as making decisions. A second useful convention is that the Foreign Office and its denizens operate in secrecy. This tradition is disapproved by advocates of open government, but I presume Sir Percy is not in this category. The nice thing about secret advice is that people are surprisingly willing to give it the benefit of the doubt. Awe-inspiring levels of wisdom are assumed in a process which, if performed in public, would probably look little more impressive than the deliberations of the Urban Council. Compare the ease with which it is possible to portray almost any American president as a bumbling buffoon, and the ageless sagacity which is held by some writers to animate the decision-making of Mr Deng and his mates. The difference is not in the quality of the decisions - it is in the visibility. Some smoke is an essential part of the magic spell. Sir Percy ought to be well aware of this. He should know as well as anyone that his public raking over old coals is making more difficult the job of the people tackling problems to which he is now just a spectator. I imagine some of Sir Percy's former colleagues are pretty peeved with him. Do we behold here an example of Burkean sacrifice to higher patriotic ideals - or do we see an obstinate old man who cannot get used to being deprived of the luxury of getting his own way? Either way, the effort is being wasted. A convincing defence of Britain's policy on Hong Kong can come only from those who were responsible - including notably Sir Percy's famous former boss. Her silence, so far, has quite drowned out his noise.