Two weekends ago at the V Festival, revellers were surprised to see a remote-controlled surveillance drone flying and filming overhead. Little to nothing was known beforehand about the drone’s use, and news reports after the fact shed little light on why or how its use was approved.

I put in a Freedom of Information Act request and discovered that the drone was part of a sales demo by a company called MW Power at the invitation of Staffordshire Police. What about the legality of the drone, I asked the police? They wondered why I was asking. Was I a competitor? Did I want to sell them a drone? It was unbelievable to the police, I suppose, that a citizen might be concerned about her privacy.

MW Power told me that more than half of Britain’s police forces have asked for a drone demo and many are finalising packages to buy the £30,000 kit – this without any public discussion about whether it is a useful way of combating crime.

Overarching surveillance infringes our privacy. So, for such an infringement to be justified, the police ought to have evidence to show its effectiveness. Instead, the police grab at invasive technologies without regard to the cost in terms of individual privacy or community trust. The police claim that drones will prevent thefts, but they can’t provide any proof. Shouldn’t such proof exist before the police throw taxpayer’s money into the sky?

Cops with helmet cameras, the DNA database, automatic numberplate recognition, CCTV – all these technologies have been slyly introduced: imagined future benefits are played up while the very tangible, immediate costs of lost privacy are airily discounted.

The Crown Prosecution Service, for example, has no figures on the success of CCTV in prosecuting crime. As for prevention, violent crime has doubled in the ten years since CCTV came to blanket the country. And yet Simon Byrne, the Assistant Chief Constable of Merseyside, still says: “People clamour for the feeling of safety which cameras give.”

I don’t. Far better to rely on real eyes in real human heads with real police officers backing them up.

But I’m told by Merseyside Police – the first force to buy a drone – that the flying spy has been “a great success and people feel they’ve reclaimed their parks”.

Has the drone’s footage been used as evidence to prosecute or arrest anyone? No. Not much of a success then.

If police forces were directly accountable to the people they serve, it’s doubtful that we would have agreed to such costly blanket surveillance – whether drones in the sky or cameras on every street corner – without the solid facts to persuade us of its necessity. But when the only person that the police have to please is the Home Secretary, then citizens’ rights are irrelevant.

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