Here’s a campaign slogan for Brian Paddick, the ex-copper who would be Liberal-Democrat Mayor of London: “The Life of Brian: He’s not a very naughty boy – he’s the Messiah.” Over the top, perhaps, but five years ago it was used in a grassroots campaign to get him reinstated as Met commander in Lambeth and Paddick ended up with a promotion to Scotland Yard.

But that was several crises ago and Paddick is now a politician without a slogan unless “I want to make a difference” counts. It is a cliché he uses with brazen sincerity more than half a dozen times during our talk.

A tall, glabrous man with a look of Gary Lineker about him, he is tense when we start talking over coffee in the hotel that used to be London’s County Hall. He looks uncomfortable in his suit as if he has spent too much time in a uniform. At first I think he is shy. Then I think he is wary of the press - having been libelled by The Mail on Sunday, which paid him substantial damages, and on the wrong end of one of Richard Littlejohn’s antigay jihads, that would be understandable. But I conclude that I am witnessing the surface ripples of an inner turmoil between his instinct to be candid, the caution he has painfully learnt and the commercial need to leave something for his memoirs, which will be published a couple of weeks before election day.

I ask him if he wouldn’t really rather be Met Commissioner than Mayor of London. “Well,” he says, “I joined the Met to make a difference and I increasingly found that I was being restricted in what I could do, until at the end where I wasn’t really able to do anything.”

Paddick, you see, is here only because of a disastrous falling out with the Metropolitan Police Commissioner, Sir Ian Blair, who did not appreciate him telling an inquiry that “senior officers” must have known that Jean Charles de Menezes was an innocent man within hours of the Met gunning him down at Stockwell Tube station in July 2005. Sir Ian moved his deputy assistant commissioner to head of paper shuffling. “The Commissioner had made it clear to me that he had lost confidence in me. The glib answer was that the feeling was mutual. Ian Blair was, for people like me, the person who was going to make the positive changes that needed to be made. To see the outcomes that we have seen is very disappointing. He does not appear to have been able to impact on the [policing] culture in the way that we hoped. He appears to be backing illiberal government ideas such as the extension beyond 28 days detention, in a way that doesn’t seem, from what we knew of the man before, to be something that he would do. We thought that he would be a liberal commissioner and yet he’s supporting ID cards.” And whatever Tony Blair wanted him to say, Sir Ian said? “Well, who knows what other requests the former Prime Minister made to the Commissioner? But he was helpful to Labour in what he said. The Commissioner of Police should go out of his way to convince people that he is not party political.”

Paddick resigned on May 31, giving only six week’s notice so that when the report on Stockwell was published he would be free to comment.

At the time he had no plans beyond taking a holiday and writing. With his police pension and his mortgage paid off, he no longer needed to work. The New Statesman offered him a column. “But even that seemed too much after all the hassle I’d had.”

Then the political offers started, not just from the Lib Dems but the Tories. He met Francis Maude, the Shadow Cabinet Office minister. It was a constructive talk but Paddick found himself agreeing with the incumbent, Ken Livingstone, who had said on TV that while Paddick might make a good mayoral candidate he could not see him as the Conservative. But nor could the Lib Dems persuade him. “I’d had a terrible 12 months at the Yard. Always in the public eye. Rumours about me and the Stockwell Tube inquiry. I thought I needed a break.” At the London Pride Festival in June he ran into Livingstone and told him he wouldn’t run.

So what happened? “For a couple of weeks I went to America with my partner. We had a wonderful time and I felt really good and I changed my mind. So I told the Lib Dems that I’d be willing to put myself forward.”

In September he was shortlisted and the next month London Lib Dems selected him. He is a Liberal Democrat by instinct, he says. He is a true liberal, keen on civil liberties and reducing the gap between rich and poor. His departure from the force abruptly ended a distinguished, if unusual, 30-year career in London policing that had began as a PC at 18. Having joined to avoid going to university, he ended up studying at Oxford, Warwick and Cambridge during career sabbaticals. In the Eighties, he was placed in charge of policing the Notting Hill Carnival. Aged 40 he was running Merton. In December 2000 he was promoted to commander of Lambeth. The next summer he introduced a policy of cautions for cannabis possession. The right wing, when they weren’t calling him the “Camp Commander”, dubbed him “The Cannabis Cop”.

“But the record shows that during the pilot we seized 110 per cent more cannabis than we had before that. Before it took so long to arrest and process offenders that some forces couldn’t be bothered. Giving constables a quick and easy way to deal with it, it enabled them to do more.”

The conservatives were out to get him. The chance came in 2002 when The Mail on Sunday paid more than £100,000 to his former boyfriend, James Renolleau, who claimed that Paddick had attended gay saunas and once had sex on the Gatwick Express. Paddick denied the sexual allegations and the more damaging charge that he had smoked cannabis with Renolleau. The MoSlater apologised on all counts, the Crown Prosecution Service announced there were no grounds for prosecution and the Police Complaints Authority decided to bring no disciplinary charge. But by now Paddick had been removed from Lambeth to Scotland Yard.

Yet even some supporters questioned his tactics. The oddest among them was, as head of Lambeth police, to sound off on a website called www.urban75.com under the name “Brian the Commander”. On drugs he sensibly said: “Bottom line, screw the dealers; help the addicts.” But he also mused that he found the “concept of anarchy appealing”, although impractical. It would presumably have sounded better in an Oxford tutorial than in a tabloid headline. “On the website issue, you know, my officers were being criticised on that website and no one was defending them, so I went on there to defend them.”

Did that seem to him a high-risk thing to do at the time? “It seemed like the right thing to do.” And was it popular? “Well, as with most things that I do, there were sort of mixed reactions really.”

On one level Paddick is a sophisticated man, an urban liberal with, thanks to his days on the beat, a grasp on life’s meaner streets. On another, he is a naïf, as the coverage generated by Brian the Commander’s thinking aloud on the web demonstrated. I think his problem is that he is cursed with incurable optimism. It would explain why he is standing as a Lib Dem for Mayor (he is now polling at 8 per cent) and also two other otherwise inexplicable choices he made in his life.

The first is that he volunteered for the police at all, one of the most homophobic organisations in Britain, probably still and certainly in 1976 when he joined. “It was a challenge,” he agrees meekly, although not one he met face on until he came out to his colleagues in the late Nineties. The other choice, reached after two broken engagements, was, in 1983, to marry. Since he has always put an emphasis on personal honesty, an ethic drummed into him by his late father, a Balham plastics salesman, it sounds like the act of a man in deep denial.

“I think at that time, naively, I didn’t feel that I was irreparably gay. For a whole series of reasons, not just the police or not even the police, I thought life would be a lot easier if I was straight. I thought maybe it was possible, not necessarily to grow out of it, but to overcome it.”

Or to live with it as a discreet part of his life? “No, that wasn’t the way forward because I had to be honest about who I was, but I felt, as I say, naively that it was something that I could overcome.”

Five years into the marriage he realised he could not and told his wife, Mary. She was “marvellous” about it, but they divorced and she has since remarried. Are they on reasonable terms? “Really good terms. When a Sunday tabloid did a kiss-and-tell story, my wife did a double page in the Sunday Mirror saying what a wonderful guy I was.”

About his current partner, he is reticent. Paddick has cashed in some of his right to privacy; this man has not. All he says is that he is a civil engineer of “mature” years (ie, not a toy-boy) and that they have been in a long-term relationship for some years. “I expect as we speak there are people trying to find out who my partner is and what he does and whatever else. And if they succeed they’ll be terribly disappointed. You learn a lot from having your private life on the pages of a Sunday tabloid. You know, and I’ve known from then on, that I had to be careful with what I do and whom I meet.”

Paddick to me plays down the sensations contained in his autobiography. But once it is published, he promises, Londoners will know all they could wish to about him. They may be more interested to know how he thinks he could cooperate with their top police officer. After promising he would not call for Sir Ian Blair’s resignation, a few weeks ago he did almost precisely that.

“I later went on to say that I thought that the Met and London would probably be better off without him as commissioner and some people have argued that there’s not much difference between calling for his resignation and saying that. But there’s a difference between expressing your personal opinion and calling for somebody to resign.”

But how could he work with him now? He says that they met again a few weeks ago. “After 25 minutes with his notetaker writing down everything he said and everything I said, he dismissed his notetaker because he realised that we were like-minded people and we were, at least for once, on the same side. I’m sure we are both professional enough to be able to work together if that scenario develops.”

I wonder if the truth isn’t actually that he would have liked his job or, if not Blair’s, at least to be chief constable somewhere, making history as Britain's first openly gay top cop. “If I had decided that my ambition was to become a chief constable rather than making a difference, then yes, but I decided on the way up that I would not shy away from making a difference. If I had decided to put my ambition as the primary objective, then I have no doubt that I could have become . . .” He stops himself. “I know it sounds a bit arrogant.”

Whether Brian Paddick will succeed in making a difference even to the duel between Citizen Ken and Bonkers Boris it is too early to say. He may, at least, change the agenda. That he is different, a different type of politician, there is no doubt.

Paddick’s plans for the capital

— Cut crime by 5 per cent every year

— Allow Oyster users unlimited bus changes within an hour for the price of one journey.

— Use money spent on temporary accommodation to build permanent social housing.