Londoners love convenience. Take the Oyster card. It is so quick and easy to waft it at barriers. A little plastic card that makes our lives simpler, the Oyster card follows in a proud tradition that started with credit cards, passed through loyalty cards and will, if the Government survives the election, culminate in ID cards.

When politicians or corporations try to persuade us to carry these pieces of plastic, they always focus on convenience. It will be so much easier to open a bank account when we have ID cards, the Home Secretary tells us. Even yesterday, Alan Johnson spoke of Labour's plans for more CCTV. We rarely hear the full story: that a central purpose is to track us, to identify our likes and dislikes and, ultimately control us.

Take an example of low-level surveillance from the world of supermarkets: the loyalty card. What has your supermarket loyalty card done for you lately? Owning one encourages you to shop repeatedly in one large store, and ignore deals elsewhere. But more sinisterly, it allows the supermarket to understand you in ways of which Mr Marks and Mr Spencer could only have dreamed. The monetary value you give away by revealing your preferences (the shopping equivalent to showing your poker hand) far outweighs the reward. It has to, or the shop wouldn't bother with the technology.

The same is true with social networking sites. They look like free software. But although they do not like to talk about it, Facebook and Twitter make their money by tracking, storing and selling information about their users. That's the Faustian pact you sign when you register. Again, we are sold the systems on convenience: here is a single place to go on the web where you can meet your friends.

London has been the world's test bench for a range of tracking gadgets. We have more CCTV than any other city - there are 250 cameras alone within a mile of my house in Hoxton. Again, convenience was at the heart of the sales pitch, this time to the police. Imagine, no more trudging the cold streets. Sit in comfort and watch the crimes unfold. You only need to get out there when there is a problem. And to citizens, the systems are sold by fear. Crime is too widespread to catch on foot, we were told. We need to use the all-seeing eye. So why did the recent House of Lords report conclude that CCTV has had little or no effect in preventing or detecting crime? The massive industry that sells CCTV to government had a ready answer — because it is not really good enough yet. The images are too blurry. There are not enough cameras. Or perhaps they need to be fitted with more advanced software (now being installed in central London stations) that can recognise your face, or even how you walk. Once the technology is perfected, then it will really start to improve our lives. Or will it?

Add to this the unprecedented raft of anti-terror legislation in the past 10 years that allows local councils and central government to monitor our emails, phone calls and travel, and you begin to see why Londoners are judged the most watched citizens in Europe. By tracking everyone to find the single terrorist, the single criminal or the single shopper most likely to buy more, we have chiselled away at each of our freedoms.

Surely some of this technology was brought in with the pure intention of making our lives better. The London congestion charge has improved the flow of traffic, hasn't it? Not conclusively. What it has conclusively done is provide a stream of real-time data directly to the police about who drives into town, how long they stay and where they go.

The NHS database is the most recent example. Millions of letters have gone out to Londoners in the past two weeks explaining that our medical records are about to be uploaded to a central database. The letters mention convenience and safety. They decline to describe the risks. The main risk is that we are moving from a system where a local GP holds records locally, accessible by a handful of practice staff, to a nationwide sump of medical data that will be open to view by tens of thousands of NHS staff, politicians, scientists and bureaucrats.

I opted out of the NHS database (no thanks to the letter, which makes opting out sound like a logistical nightmare — it isn't, by the way) because last year, two private investigators accessed my wife's medical records and used the information they obtained to track me down.

I had hired the investigators, from top London firm Cerberus, to find me. I wanted to find out whether it is possible to live a private life in surveillance London. I was making a feature-length documentary called Erasing David for the Channel 4 Britdoc Foundation. A film crew was following Cerberus as they tracked me down. I was filming myself as I ran away. I wanted to know what other people can know about me. What is out there in the public domain? Can it be used to profile me to the extent that a determined investigator, identity thief or stalker, could know what I am likely to do in the future — and catch me?

Within an hour of searching for me (all they had to start with was my name and a recent photo) the private investigators had ordered my wife's, my own and my daughter's birth certificates, and my parents' and my marriage certificates.

They ran my name through a number of profiling systems to give them my credit rating, details of property I owned and my employment history. They also ran a quick profile of me on social networking sites. I had tried to remove myself from Facebook (you can never really remove yourself from Facebook) but they were able to find a good crop of my friends. I am not particularly vulnerable, by the way, anyone could do the same to you.

Before running away, I wanted to find out what data is out there about me as an average Londoner. I compiled a list of 80 organisations — companies, government agencies, social networks — that know about me. I made subject access requests under the Data Protection Act to all of them.

The results were staggering. My desk disappeared under a mountain of paper. It turns out that the DVLA still had records of a driving offence I committed in my late teens. I am 38 and they are supposed to be deleted after 10 years. Amazon provided 120 pages of orders, friends to whom I send presents and even things that I might be interested in, based on my previous browsing.

Transport for London reluctantly sent me a terrifying log of every crossing in and out of the congestion charge zone I have ever made. I had bought a low-emission car to avoid the charge, but they track it anyway. When I called them to ask why, the bemused manager said that the police might need it if I got myself in trouble in the future. “It's not a police state or anything,” he reassured me.

My bank sent me records of my phone calls. It had lost a cheque in 1997, and the transcript read like a Stasi file. “Mr Bond seems angry. His voice is raised. And he is considering leaving the bank.” Tesco knows what food I like. I suppose that's not a surprise. But it also has me pegged as, among other things, a new dad, who buys beer on a Friday, and sometimes a little more than average …

In itself, this data was unsettling but what really gave me the fear was when I called these people back and asked them to delete the data. “Do what?” was the standard response. “Delete it, please.” “Oh no, we don't do that.”

And the penny dropped. Knowledge is power. For governments, this means control, for companies, profit. Once we give this stuff up, we are never getting it back and it sits around forever.
While researching the film, I visited Professor Ian Angell from the London School of Economics. He is a world expert on databases. I put it to him that I have got nothing to hide, so why should I have anything to fear? Well, on average two per cent of all databases are wrong, he told me. So if you've got nothing to hide, then so long as you're in the lucky 98 per cent whose data is not corrupted, not mishandled, you have nothing to fear. But the other two per cent have nothing but trouble in store.

This is the tip of the iceberg. We are now beginning to see early victims of our brave new database world. There will be more. The next time you hear someone in a company or in government say how much more convenient a new system will be, I guarantee somewhere along the line it will be harvesting your data.

Erasing David is in cinemas nationwide from tonight and on More4's True Stories on Tuesday at 10pm; erasingdavid.com.