Andrew Mitchell actually had big doubts about becoming chief whip. True, he had often spoken to friends of being ‘a whip at heart’, and how he’d loved the army-like camaraderie, discipline and intrigue of serving in the whips’ team under John Major. But his regular trips overseas as Secretary of State for International Development meant he didn’t know many of the Tory MPs elected in 2010.

And he so enjoyed being Development Secretary, too, and making a visible impact in one of the few ministries not afflicted by cuts. But Mitchell’s appointment as chief whip this time last year suddenly thrust him from the margins of Westminster to the heart of government, advising Cameron on other promotions.

Two weeks later, on Wednesday 19 September, Mitchell spent much of the day placating colleagues who’d lost their jobs. At 7.31 p.m. that night, Mitchell left his room in the Cabinet Office, mounted his old-school bicycle (complete with undergraduate basket), and pedalled his way down Downing Street. He was late for a dinner at the Carlton Club. At the gates, Mitchell was stopped by the police, and told to wheel his bicycle through the side entrance instead. He protested; he swore, and then?

The next night the Sun’s headline screamed, ‘CABINET MINISTER: POLICE ARE PLEBS’. The paper claimed that Mitchell had told the police they didn’t run the ‘fucking country’; that they were ‘morons’; and, most politically dangerous, that they were ‘fucking plebs’. The media went into a frenzy, egged on by the Police Federation. Mitchell denied uttering the phrases, but most people in politics felt the Sun story smelt right. And the case against Mitchell looked clear cut once the Daily Telegraph printed a ‘police log’ which contained similar phrases. Plebgate, as it became known, fitted the image of the Cameron regime as a bunch of arrogant, public school toffs. Mitchell toughed it out for three weeks, but was eventually forced to resign.

That might have been the end of it had an independent TV company, Blakeway, not managed to get hold of CCTV footage which told a very different story. The black-and-white images, from three cameras, suggested Mitchell was the victim of a terrible conspiracy. One which, a year on, the police have still not got to the bottom of.

The evidence is clear enough. The police ‘log’ reprinted in the Daily Telegraph cited ‘several members of the public’ who were present and ‘visibly shocked’ by Mitchell’s behaviour. But the CCTV showed no such crowd. While investigating for Channel 4’s Dispatches, Blakeway also saw two emails from a man who claimed to be a bystander among the supposed crowd. He had complained about the ‘incident’ to his MP, John Randall. Coincidentally, Randall was Andrew Mitchell’s deputy chief whip, so the email was soon shown to David -Cameron.

When we tracked the emailer down to his home in Ruislip, the man claimed he hadn’t been ‘a witness to anything’; and when I questioned him on his doorstep, he denied any police links. When we obtained his marriage certificate a few days later, however, it revealed he’d been a policeman. By now, the Met knew we were making a programme for Channel 4. They quickly arrested the man in Ruislip, and announced that he was actually a member of the Diplomatic Protection Group, the body whose work includes guarding Downing Street.

The police added, though, that the man from Ruislip wasn’t working in Downing Street on the night in question — which suggested collusion. One of the most senior figures in government, it now seemed, had been stitched up by some of the police whose job it is to protect the most powerful people in the land.

David Cameron angrily demanded an inquiry and 30 officers were assigned to Operation Alice, which even some of Mitchell’s friends thought was a bit of an over-reaction. The Met urgently needed to restore confidence in the police. But this didn’t happen. The police extended, extended and — a year on — are still extending that investigation. We still don’t know what happened.

Initially, the Met Commissioner, Bernard Hogan-Howe, promised a ‘ruthless search for the truth’. He told MPs he hoped to report to the Crown Prosecution Service by the end of January. But January came and went and, two days before we broadcast a second Dispatches programme, the Met arrested two more officers from the DPG, on suspicion of misconduct in a public office. In March they sent an interim file to the CPS, though it seems to have been pretty thin.

Unfortunately, this move was accompanied by what seemed to be a leak from the Met, stating there was ‘no evidence that Downing Street police officers lied’. MPs on the Home Affairs select committee have asked the Commissioner whether he himself was responsible for this leak. Only days before, Hogan-Howe held several routine briefings with journalists who reported this line. What was the Commissioner up to? And why was his investigation taking so long?

One reason was the strange decision taken by Deputy Commissioner Pat Gallan (who runs Operation Alice) to interview over 700 members of the DPG — what seems to be the entire staff. To Andrew Mitchell’s allies, this decision stretched the notion of a thorough investigation to absurd lengths.

It’s hard to see how the Met comes out of this well. There appears to be strong evidence that several officers in sensitive positions lied in order to smear a senior member of the cabinet and to have engaged in some form of conspiracy. Once exposed, the affair could have been handled speedily and efficiently in order to protect the wider credibility of the police.

Meanwhile, Andrew Mitchell’s career remains on hold. There’s talk of a return in the pending reshuffle, or of him becoming Britain’s next European Commissioner in 2015. David Cameron thinks he’s been treated unfairly, but until the Met finishes its task it may be hard for the PM to rectify that injustice.

Since our first Dispatches programme last December, eight people have been arrested — five police officers from the -Diplomatic Protection Group, and three civilians. The officers have been suspended from duty, pending possible charges. Operation Alice has taken up a huge amount of police time and money (£144,000 by May). Yet it surely ought to be a relatively simple inquiry compared, say, with a fraud case. Perhaps the police can’t crack the case. Or perhaps, equally worrying, they don’t want to.