Sir Ken Macdonald takes the firm view that prosecutors should be in the public eye — not hiding in the backroom. But the recent flurry of publicity about his relationship with an attractive blonde prosecutor is not quite what he had in mind. True to form though, the coverage has not forced him into hiding. It is very much business as usual — although that topic is firmly off-limits. “I don’t talk about my private life or anyone else’s,” the Director of Public Prosecutions (DPP), says. Nor will he confirm, or deny, whether the matter is now history.

It is not the only subject that is off-limits. Sir Ken, who last year had his contract renewed as DPP, is careful not to touch on such hot potatoes as a ministry of justice — “it won’t impact on our work” — on sentencing reform or on the role of the Attorney-General.

That political adeptness ensured that he stood aside at the very start from any decision on cash-for-peerages. “It was nothing to do with Tony Blair [he insists that he does not know the Prime Minister well]; it is Cherie,” he says. “She is the wife of a potential witness. If a potential witness in any case I had to deal with was the husband of someone I’d known, I’d take that decision. I was in chambers with Cherie for three years; obviously at that time we shared a commercial interest.”

The decision therefore falls to Carmen Dowd, head of the special crime division. Despite the insistence of Lord Goldsmith, QC, the Attorney-General, that he will be consulted, it is clear that Sir Ken will not allow it to override what the CPS decides. “The decision will be made here,” he says. “If the Attorney wants to be consulted, of course he will be consulted. But we will make the decision.”

He is on record as criticising the drip-feed of leaks on the police investigation — “not helpful” — but will not point the finger. “I just think it should stop.” As to the time the inquiry has taken, from the Crown Prosecution Service (CPS) end, he says, files are dealt with expeditiously each time they are handed over. “One thing I can say is that we will be as transparent as possible. There’s a lot of pressure floating around. We always knew there would be... to come down on one side or the other.” But if there is a case for prosecution, it would go ahead. If not, it would not, he said. “In a case such as this, we are not going to be influenced one way or the other. We do a lot of cases like this, where you are damned if you do and damned if you don’t... that’s a strength, in a way. Our prosecutors are not terribly fussed by it. I don’t think Carmen Dowd is fussed by it.”

At a time of such politically sensitive cases, Sir Ken’s role in promoting the independence of the CPS is crucial for public confidence. Where his boss, the Attorney-General, may struggle to persuade the public that he can separate his role as legal adviser to ministers from his public interest role as chief law officer, the DPP can point out that he is not a politician. And while the Attorney finds himself under attack for injunctions over reporting cash-for-peerages details, Sir Ken can be on side with the media over the reporting of terrorist trials.

His recent backing of a successful challenge by the BBC and The Times on reporting the trial of Dhiren Barot marked a change in policy. Openness should benefit not just defendants but the public, he says, who are “entitled to as much information as possible” and to hear everything unless there is good reason for it to be withheld. “I have ended the widespread practice of the Crown remaining neutral when defendants apply for reporting restrictions in trials; it should be rare for the Crown not to have a view. And there will have to be a compelling and fully articulated reason before we support the exceptional step of banning or restricting coverage in criminal cases. Terrorist cases are obviously prominent in this category.” He has also agreed a protocol with the media (backed, he points out, by the Attorney) on what police evidence can be disclosed. The change has meant regular coverage of such trials, rather than just the opening and verdicts — a change that is “good for public confidence”.

In part, the changes stem from Sir Ken’s career at the Bar defending people charged with terrorism. Juries, he says, have to be trusted. In a technological age they can google information about defendants. “But in my experience, when the trial is under way, they focus on the evidence before the court. I used to make these arguments to stop cases [that coverage had prejudiced the chance of a fair trial]. But it was really an argument against the jury. I used to feel uncomfortable about it. If you don’t trust juries to put things out of their minds, that is fundamentally going to undermine jury trial.”

Two weeks ago he appeared before a joint Lords-Commons select committee to repeat the case for intercept evidence to be used in terrorism trials. But, despite strong backing from both himself and the Attorney, there seems to be a stand-off. There are concerns from the security services and telephone companies about bureaucracy; about terrorists learning what techniques can be deployed; how much information is being handed over; and risks to staff who may give evidence. “Terrorists,” he says from his defence experience, “take the view that they are being tapped all the time anyway so they just keep on talking.” The issue looks landlocked. “There are strong feelings on both sides but I hope that we can find a way through.”

He has 18 months to go in the post so it is too soon to be thinking about the future. He is enjoying the job and his agenda is to make the CPS a “world-class prosecuting service” at the centre of the justice system, with prosecutors not only charging but running their cases as advocates and giving cautions to divert offenders from court. But he is unlikely to follow his predecessors and become a judge. “I would like to do something academic — perhaps combined with work at the Bar.”

Meanwhile, as DPP he will continue treading the fine line of being an “advocate for human rights” while tackling threats to national security. To protect both, he argues, is “ourduty as lawyers”.

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