Who watches the watchmen? Or, to translate Juvenal another way: who polices the police? The answer this week was a New York fund manager, of all unlikely superheroes, who provided the Guardian with key footage of the minutes leading up to the death of Ian Tomlinson during the G20 protests in London. The man came forward because "it was clear the family were not getting any answers".

If there is anything to feel optimistic about today, perhaps it is the hope that we are witnessing the flowering of an effective inverse surveillance society. Inverse surveillance is a branch of sousveillance, the term coined by University of Toronto professor Steve Mann, and it emphasises "watchful vigilance from underneath", by citizens, of those who survey and control them.

Not that turning our cameras on those who train theirs on us is without risk. Indeed, one might judge it fairly miraculous that the man was not forcibly disarmed of his camera phone, given that it is now illegal to photograph police who may be engaged in activity connected to counterterrorism. And as we know, everything from escorting Beyoncé to parking on a double yellow while you nip in to Greggs for an iced bun can now be justified with that blight of a modern excuse - "security reasons".

Yet it will by now have dawned on even the most dimwitted Met officer that it is increasingly impossible for them to control the flow of information about their activities - to kettle it, if you will - no matter how big their army of press officers putting out misleading information in the immediate aftermath of any event may be.

Did the Met genuinely think they could prevent the emergence of a far more joined-up picture of Tomlinson's passage through the City of London that afternoon, much as they thought they could suppress the details about Jean Charles de Menezes's tragic final journey? If so, their naivety is staggering.

Yet it's odd how often it has been the little ways in which the state attempts to keep tabs on our behaviour - tracking devices on wheelie bins and the like - that have most alienated those who previously bowed to authority. Also captured on film and published yesterday was an amusingly British act of defiance - a pyjama-clad householder blocking dustmen into his road by standing in their path, after they had declined to empty his neighbour's bin of five pebbles.

As Tomlinson's death shows, though, it's not all Victor Meldrew-meets-Passport-to-Pimlico larks. Indeed it is something of a shame that certain elements of society have only recently woken up to the possibility that the police might not be the faultless, justice-dispensing force of establishment myth, and only because - in the cases of De Menezes and Tomlinson - they have seen it with their own eyes, or at least enough of it to provoke a suspicion that was hitherto absent.

The serially deferential dismissed the Blair Peach outcry as lefty agitating. They did not make a point of seeing Injustice, the brilliant and desperately depressing 2001 documentary about deaths in police custody, of which at the time there had been 1,000 in the previous 30 years, without a single conviction.

But they are undeniably more cynical and inquisitive now, and it is interesting that for many previously deferential Brits, the Countryside Alliance march a year later, in 2002, was such a watershed. Here, peaceful marchers who considered themselves fine, upstanding members of law-abiding communities, were genuinely shocked and appalled at the manner in which they felt police treated them during the demonstration.

It is hard to say whether this sea change in the amount of trust people are willing to put in their alleged protectors will be reflected in the judgments of those with the power to call those protectors to account. The De Menezes jury chose notably to believe the civilian witnesses who countered the police line and said that officers had not shouted "armed police" before they shot.

Then again, the Independent Police Complaints Commission had apparently failed to interview the police officer who attacked Tomlinson 48 hours after he had come forward, with anonymous Met sources briefing that the man had not known it was him till he saw the footage, and collapsed upon realising it was. It is up to you how you interpret that memory hole. Maybe the attack was merely a forgettable instant in a trying afternoon. Maybe he had seen so many lone men walking with their hands in their pockets truncheoned that day that his own crack of the baton didn't stick in the mind.

Either way, perhaps the IPCC should interview the officer no matter what sort of funk he is in. After all, from what little we know of him, he would surely agree that there are no excuses for dawdling.

But we have no means of chivvying the IPCC along, alas - of giving them a metaphorical shove in the back, or a notional truncheoning. So in the meantime, let's note that a day which started out protesting about a very different them-and-us situation has reminded us that there is more than one attritional show in town. And sometimes, New York fund managers are on our side.

marina.hyde@theguardian.com