Guilty until proven innocent: White, middle-class Eric Clark was all for the police, until they knocked on his door

THEY could, as one of the detectives pointed out months later, 'have come around at three in the morning and kicked your door down'. In fact, they arrived at 3.40 on a February afternoon, waving warrant cards as they swept through our front door, while two of our three children, aged 15 and 13, were alone in the house. They told my daughter to phone me at the office but not to reveal that they were policemen.

'Just say there's two men to see him,' said one.

'No, no - just one man to see him,' said the other.

It was not the first time my children had seen policemen in our Sussex home. Some are friends. As a journalist and a thriller writer, I have had close police contacts for 30 years. I've lectured at the Police Staff College and been a member of an Inspector of Constabulary's study group. In arguments about the police, and the powers they should have, I have generally been on their side.

But this was different. I was suspected of crimes which apparently involved forged passports, drugs and illegal immigrants.

It had begun, I was to learn, last November. Rachael, our eldest daughter, wrote from university, asking for her birth certificate in order to apply for a student loan. We could find only a photocopy. So I wrote to Westminster Register Office - the area where Rachael was born - asking for another certificate and enclosing details taken from the photocopy. A few weeks later, Westminster replied. They could find no trace of Rachael's birth. Had I further information? I sent a copy of the photocopy. Weeks later, I had heard nothing. I began a letter of complaint. Halfway through, I noticed the year of birth on the photocopy was 1973; in fact, Rachael was born in 1975. Westminster had made a mistake 19 years ago and we had never noticed.

I rang to explain. 'I shouldn't even be talking to you,' the Registrar said. 'Where did it (the certificate) come from? It was not from here.' She had called in the police. She would say no more.

Friends explained what must have happened. Rachael had photocopied her birth certificate, altered the date with Tip-Pex and then re-copied. Kids did it all the time, we were told, to get into clubs or pubs. (Rachael, it turned out, had planned to go youth hostelling at 13, but had never actually used the copy.)

Then came the February visit. When my wife and I met the two detective constables from the Met, they extracted the birth certificate copy from a file two inches thick. One stabbed his finger at an eight-digit number in the top left-hand corner. It showed the source of the certificate, he said. It proved it was one of a batch of 200 stolen from Haringey four years before Rachael was born. How did I explain that? The questions came harder. How could a father not notice the wrong birth date? 'Facts' shot out. Stolen birth certificates normally didn't emerge for 20 years; then they were used for all sorts of criminal practices, such as obtaining passports and arranging false marriages.

All this came from the leading detective, who had emerged as Mr Hard Guy. His sidekick was Mr Jokey - the technique will be familiar to any movie-goer. Could Rachael be contacted, he asked. Yes, we said, but the phone was communal. Mr Jokey knew all about students. They were always stoned on drugs. 'I know the kind of thing. Some guy answers the phone.' He began rolling his eyes, pretending he was weaving around. 'Yeah man, she's down the corridor, I don't know where, we're all stoned here.' The idea, presumably, was to goad us into saying something we might regret.

Eventually, Mr Jokey confided: 'If we hadn't talked to you, if we hadn't seen, if we'd just gone on the files, we'd have had you away by now. No question. You'd have been down the block.' He turned to his colleague. 'Are you going to leave it there?' A long pause. 'Yes - for now.'

It took six months from beginning to end before the police officially dropped the matter. This included a formal questioning of Rachael and the help of two lawyers and a private detective. During the inquiries, I understand, post was diverted, telephone calls monitored (though not tapped), my bank accounts probed.

The mystery of how we were issued with a 'stolen' birth certificate remains. The Registrars concerned are both dead. The private detective has an explanation, but it is guesswork. In 1975, the Westminster Registrar runs low on forms and unofficially 'borrows' some spares from Haringey. Nothing is written down. Years later, a clerk notices that 200 numbered certificates have gone missing and reports them stolen.

And the lessons? Obviously, nobody should alter birth certificates. But there is a more general one. It can happen to you. I am white and middle-class, with a stable family life, a very fixed address and some knowledge of the law and police practice. What would have happened if I had been none of these things? Although I can understand the police behaviour - I could have been a dangerous villain or the link to one - I still feel disquiet. The arguments for 'unfettering' the police no longer impress me: only when you are involved do you realise just how great police powers already are.

Rigorous protection against police questioning is vital. Faced with an unexpected and frightening situation and with skilled questioners, it is all too easy for an innocent person to panic: to lose their temper, make silly remarks.

One group of parents routinely tells their children: never answer questions if you find yourselves involved with the police, without your parents or a solicitor present. As one told me: 'It's so easy to say something you don't mean. It's not all like The Bill, you know.'

Those parents are the police themselves.

The author's latest novel, 'Hide and Seek', is published by Weidenfeld and Nicolson.