He has interviewed presidents and prime ministers from Washington to Warsaw. He walked with history, reporting those fateful days when Mikhail Gorbachev ordered the Red Army to stand back and let what remained of international Communism crumble around its barracks. And now Martin Sixsmith, 43-year-old former star BBC foreign correspondent, is on an assignment more difficult than ever were pieces to camera against the backdrop of a bullet-strewn plaza. For the past five hectic and leak-strewn weeks, he has been director of information at the Department of Social Security - the man on whom Harriet Harman's political fate may hinge.

Flakking for HM the Queen or Mohamed Al Fayed may look like hot seats, PR-wise, but they hardly compare in discomfort - and poor pay - with the Civil Service job that Sixsmith has undertaken. For the most difficult thing he has to do, since above all else he is the spokesman for a department rather than just the minister's man, is serve Harman well while making sure he can survive to do the same for her successor, if and when the axe falls.

That's a hard task in these tense pre-Budget days. Around him the sharks circle, lobby journalists and editorialists out for Harman's blood. There is the irascible Alistair Campbell, de facto head of government information, to worry about. Sixsmith has meanwhile to ingratiate himself with the permanent officials in his department; on them he relies for his raw material. Then there is his Secretary of State, needy, impulsive, not always the best judge of her own political interests. And, of course, there is also Frank Field.

The past week has, to put it mildly, been eventful, a swim of late-night phone calls to (select) lobby correspondents and front-page stories with spin. The tide is unlikely to abate - you cannot do welfare reform without the DSS. Mr Sixsmith's lack of experience not just of print media but the avaricious and herdlike instincts of the British press, right-inclined and loud of voice, means he is having to learn a lot, fast. But by all accounts he is a quick student (who studied Russian poetry at Harvard as a postgraduate) and has enough sense of proportion and depth of personality to see him through the bad times.

So what does this poacher-turned-gamekeeper bring to the task? I would have liked to ask him in person, but ministerial sensitivities were running high this past weekend. This information officer was not able to provide information about himself. No one doubts Martin Sixsmith's intellect - once upon a time he even passed the stiff exams for the Foreign Office, but turned them down to join the BBC. There his career ran well till, they say, his Washington tour, when he began to receive the consummate managerial skills of the then managing editor of BBC News, Chris Cramer, now a personnel director at CNN. During his second Moscow tour family pressures grew (he has four children), prompting a return to the United Kingdom, albeit without a secure berth.

The DSS information job fell vacant last year when there was a chemical imbalance between Steve Reardon, a career information functionary, and the new regime. There were not sackfuls of applicants, partly because the onerous post has a relatively low grading. In the competition Sixsmith shone: in the rigorous process, his experience of BBC bureaucracy, bureau management and the public service ethos impressed selectors. His was the single name presented by the system to Harriet Harman, who as Secretary of State had the right to veto (causing a new selection procedure to start) but nothing else.

Whitehall has since looked sympathetically on. The DSS, despite appearances, is not a "political" department in the way, for example, the Home Office has to be. Its civil servants are not used to the rough and tumble of front-page controversy, nor the fine arts of presentation. Sixsmith is thus fighting on two fronts, projecting a minister who is under siege, while also convincing sceptical colleagues that the department needs to sell itself better. He has enormously complicated subject-matter to master - benefit tapers, eligibility criteria and all that - along with some subtle inter-departmental relationships (notably with the controllers of spending in HM Treasury).

Yet BBC friends of Sixsmith say he would not have joined the Government Information Service in any other role. Welfare reform is a make-or-break issue for this government and he is in there at the kill - driven by an intellectual commitment to the challenge rather than to any personal political agenda. "We'd die for the kind of access Martin has," said one source, though he did not add that if the BBC had a real grasp of political and policy priorities it would ensure that more of its journalistic stars did gritty coverage of social security rather than glamorous two-ways outside the Kremlin.