114 people were pre-emptively arrested in the midst of a plan to shut down Ratcliffe on Soar power station in April 2009. Despite not being charged, most were given restrictive bail restrictions. Proceedings were dropped against all but 26. One of those not prosecuted was a protester called Mark Stone, who was in fact an undercover police officer called Mark Kennedy.

The problem for the police is that Kennedy had helped to plan the action, had paid for the vehicles to transport the equipment on the day. When police vehicles were spotted at the power station and activists debated calling the whole thing off, Kennedy drove out for a recce and reported that the police had gone, putting the action back on. He encouraged others to join in.

Whatever the Met's guidelines are for involvement of undercover officers, you have to wonder whether instigating, planning and inciting crime are outside the remit.

Last October Kennedy's true identity was discovered by activists. He professed remorse, though notably didn't tell activists much that they didn't already know.

The Ratcliffe trial went ahead for 20 who admitted planning the action but claimed to be justified as they were preventing a greater crime. Last month, they were found guilty, with the judge leniently sentencing them amidst glowing praise for those convicted.

The trial of the remaining six - who said they had not decided to go on the action at the point of arrest - collapsed today.

Trial collapses after undercover officer changes sides says the BBC, with the same words appearing in the Daily Mail, Telegraph and across the news media.

'The undercover cop who turned' is a great movie premise (actually, it's a mediocre and somewhat corny movie premise), but it's not what happened. He had swiftly retracted his tentative and unspecific desire to help the defendants. It was only when the defence lawyers insisted that there must be witheld documents about Kennedy's part in the action and asked for disclosure from the police that the Crown Prosecution Service dropped the case.

So, it was not Kennedy's work for the activists that saved the day but his work for the cops. The opposite of what the headlines say.

It is reasonable to assume that Kennedy reported to his superiors about the action. Indeed, there has been reference to a report planned by NETCU for ministers about it that presumably includes material from their man on the ground. Yet none of it was disclosed by the prosecution.

Rather than expose what they knew the Crown chose to drop it. After hundreds of thousands of pounds had been spent and six, if not 26, people had spent nearly two years preparing for prosecution for a crime which the state knew they didn't commit.

Imagine if a police officer sees a street fight. Ten minutes after it's over a vanload of cops turn up. Yet at the subsequent trial, the prosecution don't even mention the one officer who had actually seen the event. There'd be the pervasive odour of rat.

The CPS said they'd found

Previously unavailable information that significantly undermined the prosecution’s case

and specifically said it was

not the existence of an undercover officer

It was just coincidence that they only found this mysterious evidence, 21 months after the incident, within 48 hours of the prosecution's request for the Kennedy documents. When your job is to secure convictions, it must be tempting to withold evidence that exonerates.

Had the prosecution not been aware of Kennedy's outing, those six people would tonight be preparing for their second day of a three week trial at the end of which, like the 20 last month, they would probably have been found guilty.

Yet the way the story's being told totally misses this stuff. Almost as a mirror of the Ian Tomlinson killing being One Bad Cop, this is One Good Cop gets the activists freed. As with the Tomlinson case, it avoids the real issue - this is institutional. Kennedy didn't do anything to help the defendants; the police and CPS chose to conceal evidence as a matter of policy.