News that muggers are deliberately targeting voyeurs has lifted the lid on a world of illicit sex. Tom Spender reports

There are plenty of cars in the Scratchwood car park at 5pm on a Saturday afternoon.

Nothing unusual there, you might think. After all, Scratchwood is a nice spot.

What is strange is that there are middle-aged men just sitting inside these cars - and they aren't there for the great outdoors.

These men are cruising for sex. For this unobtrusive car park on the edge of Edgware is one of London's top sites for 'dogging' - a fad in which people watch other people having sex in cars. It is also a popular haunt with gay men looking for casual encounters.

There are lots of different types of cars - a sporty red convertible, a four-wheel drive and an expensive-looking saloon are just some of the vehicles in the car park.

Nothing much seems to be happening. Occasionally someone will move from one place in the car park to another, but no-one is talking.

Then a yellow saloon reverses into the space next to mine. There's a middle-aged man looking across at me. I feel uncomfortable.

I get out of the car, go around to his window and ask him what he is doing in the car park.

"I don't know," he smiles. "What are you here for?"

An expensive convertible pulls up on the other side of my car, with a bald man sporting a goatee beard inside.

Both are gay men looking for partners. They are friendly and say that, despite recent police warnings, trouble is rare at the car park.

"You have to be careful not to get blocked in in your car. You get trouble when there are only one or two cars in the car park. Once a group of men tried to frighten me - giving it a bit of verbal, revving their engine and zooming up and down in front of me. I reported them to the police. The police come up here sometimes - they just tell us to be cautious," says the goateed man.

According to the web site Swinging Heaven, dogging is predominantly a British activity. It says the term originated in the 1970s to describe men who spied on couples having sex, 'dogging' their every move.

Today, some couples encourage the voyeurs to watch and, in some cases, join in their lovemaking. According to the web site, the signal for voyeurs to move in on a car in which a couple are about to have sex is a flash of the car's interior lights.

Academics have said that text messages and the internet have fuelled such activities, making it easier for people to get information and arrange meetings. Some of the web sites have thousands of members.

Now Barnet police are visiting the web sites - to post warnings that people who frequent Scratchwood are being targeted by muggers.

Detective Chief Inspector Dick Henson said the people behind sites such as Swinging Heaven had been extremely co-operative in helping to get the message across that victims of crime will not be persecuted by police.

In Kent, health officials have become concerned that unprotected sex during dogging encounters is spreading sexually-transmitted diseases, such as chlamydia, syphilis, hepatitis and HIV, which are on the increase.

They too are using the web sites to urge participants to use condoms.

Back in Scratchwood, the two gay men I met say it has been a popular cruising spot for several years and that car parks have become more widely used because many public toilets - used as cottaging sites - have been closed. During the day, it is mainly gay men who go to Scratchwood in search of sex - the heterosexual couples arrive after dark, they say.

At midnight the car park is busier than before. A constant stream of cars enters and exits the car park, with about 15 in it at any one time. Again it's all single men. Periodically, some of them leave their cars and disappear into the bushes.

The bearded businessman returns as a four-wheel drive with about four men in it drives into the car park. "You do have to be careful," he says, eyeing the jeep.

Scratchwood is not the only dogging and cottaging site in and around the borough.

The Swinging Heaven web site also has messages advertising meetings in a car park in Warren Lane, just north of Stanmore. In Kingsbury, the Fryent Way Open Space is particularly popular with young Asian swingers.

Hampstead Heath, just south of Golders Green, has long been a favourite haunt of cottagers - and the muggers that prey on them. And near to Scratchwood, on the other side of the A1, the Nut Wood car park is also said to be popular.

Not tonight though. As I enter Nut Wood, a couple are returning to their car. There is one other vehicle across the car park, with a solitary man inside.

I leave him to it.