It is a beautiful, sunny day at the open-air cafe at Kenwood House, in north London, but, even here, Big Brother is watching. The Cyclops eye of a CCTV camera (one of more than 4m nationwide) clocked the tele-vision writer Peter Berry and me coming into the park, and clocked us going out. Presumably, it and others in the park know how long we stayed, where we went and what we did. There was a time when this might have seemed like a good idea, what with the terrorist threat and all. But Berry’s new five-part thriller, The Last Enemy, may make you think again.

Arriving with what seems like precision timing, The Last Enemy is a pacy contemporary thriller going out on BBC1 in the same Sunday-night slot in which State of Play gripped the nation. It explores the dangers of the surveillance society – how much we want to be watched, and how much we want to watch other people. It is set in the near future, but much of what it is about is already here.

It is not just recent high-profile outrages such as the secret bugging of a Muslim MP, or thetapping of prisoners’ telephone conversations with their lawyers. The problem is, we are all being watched all the time. Every journey in every car is monitored, passport-control security at Heathrow scans irises, children use fingerprints to access school libraries and meals. Commit a crime and any local authority can bug your phone. With 600 public bodies having the legal right to ask, there are 1,000 requests daily for covert surveillance or access to private information. Britain is, according to the shadow home secretary, David Davis, the most spied-on country in the free world.

With the would-be energy and pace of the American series 24, The Last Enemy is a cautionary tale about where such a technology-driven society might lead. It tells the story of Stephen Ezard (Benedict Cumberbatch), a brilliantbut reclusive mathematician who has obsessive-compulsive disorder. He returns to Britain after several years when his brother (Max Beesley) is killed, and finds that it is an ID-demanding, card-swiping, body-searching nation where surveillance is omnipresent.

For Berry (who also wrote Prime Suspect 6), the germ of the idea was planted when he saw two men at Euston station. “They were businessmen, in overcoats, and one was larger than the other. The larger one picked a piece of lint from the smaller one’s coat, as though he owned him. The other one looked completely powerless.” For Berry, this was a metaphor for identity cards. While The Last Enemy is not about the rights and wrongs of ID cards, the threat that the technology behind them will get out of hand makes for high, perhaps paranoid drama.

“The idea of having to account for yourself to someone who has power over you is so appalling,” Berry says. “You may not have to carry it, but if you don’t, you will have to report to a police station within 24 to 48 hours. I don’t want to live like that.” Anyway, like all technology, ID cards are open to fraud. The series demonstrates how to forge fingerprints for £10.

The Last Enemy is not, however, a Luddite rant. “One of the things Peter has done is to present strong arguments about why surveillance technology is a good idea, and why it would be great if we could stop the bombs,” says the producer Gub Neal, a co-creator of Cracker and producer of many of the past decade’s most acclaimed TV dramas. “Eva Birthistle plays a brilliant young politician who putsforward all the right arguments about why we need to offer people a greater sense of security. But she steps over a line. What is frightening is how easy it is to do that.”

The series has made many members of the cast think twice about the capacity of technology to worm its way into every corner of your life. David Harewood, who plays a high-ranking intelligence officer, was one of the 25m who had data lost when HM Revenue & Customs mislaid its infamous discs. “That really scared me – my children’s data is who knows where.” Even the number of traffic violations he is accused of is beginning to worry him. “I’m starting to think my name must be on some list, or that they are watching me 24/7. I find the level of surveillance amazing.”

Cumberbatch is also feeling twitchy. “I think we are being sold an utter lie with the idea that security means safety,” he says. Drinking black coffee during filming in north London (“Hold the sugar, unless it’s demerara”), he is an engaging character who talks so fast, he doesn’t always have time for all the words in a sentence. It makes him sound like an overedited surveillance tape, but his loquaciousness is not the only characteristic he shares with Stephen Ezard.

“My girlfriend had a good laugh when she found out I was doing this, because, although I’m not OCD, I have been known to check my temperature and worry too much about symptoms. And I do have threshold anxiety. I have this thing where I have to check the gas is off two or three times.” Usefully for this part, he has always felt something of an outsider. His grandmother paid for him to go to Harrow school, where, rather endearingly, he “fell in with a nice bunch of teachers”. They inspired him to act, but didn’t help him feel more like his affluent classmates. “Everyone was always going on fantastic holidays, and I would be like, ‘Yeah, I’m going to see my gran in Brighton.’”

Cumberbatch is getting used to playing unnervingly intelligent characters, having played Stephen Hawking in a BBC drama. “It’s a bit of a stretch, even for your memory sometimes. The other day, I had a vast tract of dialogue, and Peter, the bastard, had written quite complex lists into it. Stephen Ezard is an awkward, accidental hero, but he is also running around handling guns, coming across bodies, escaping bullets and policemen, living underground, being tortured, being driven around in the boot of a car and having a passionate sexual affair with his brother’s widow. There’s a vitality to him.”

There is also a dark secret at the heart of this story, being kept by the enigmatic Barbara Turney (Geraldine James), a senior security figure with an ambiguous role at the heart of government. Sitting in one of the unit buses during filming in London, Gub Neal talks calmly about some frightening scenarios. “Identity cards will work up to a point, but they will get lost or forged. The next step up is an intravenous tag, which people carry around from birth, and which can’t be removed unless we take off a limb.

“Or, even better, there’s a pathogen that attaches itself to your DNA. What if you can then introduce an ID tag that has a corrective effect on individuals, or that reacts in different ways depending on your ethnicity? The possibilities are endless.”

Many weeks later, away from the set, it is easy to dismiss these ideas as paranoia. Then a strange thing happens. Berry had lent me a 38-page dossier about the science and technology of surveillance, which a researcher had compiled for him (and had delivered in a brown paper envelope outside Tufnell Park Tube station). When I returned it to Berry using the less cloak-and-dagger services of the Royal Mail, it seemed to get lost in the post. And when he finally received it, many months later, it had been opened.

Have the secret services been reading his mail?

The Last Enemy is on BBC1 on Sunday, February 17

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