When is a television set not a television set? When the owner has been caught without a licence...Not very funny, but being rumbled by TV Licensing investigators is no laughing matter, especially when a £1,000 fine is at stake.

Despite the harsh penalty, some people refuse to buy a licence and those who are caught come up with some bizarre reasons for not forking out.

"There have been cases when people try to pass the television off as something else, like a microwave oven or even a fish tank," said Jane Lodge, regional communications manager for TV Licensing.

Detection van manager Phil Carvill operates the computer system, called Lassy, in the back of one of the vans. It works by picking up signals from oscillators in television sets - wherever they are

"Men have been known to say that they thought their wives had bought the licence and parents have said that it's only their children who watch the television.

"One man even covered his set with tin foil, thinking we wouldn't be able to detect the signal, while others hide them in places like the bathroom, fridges, laundry baskets and wheelie bins.

"Another man put a table cloth over the set but forgot to cover the video, which meant he must have had a television. Our officers have seen and heard it all."

TV Licensing is currently operating a 'stealth campaign' in Oxfordshire, which means it is not disclosing where and when investigators will operate.

The aim is to reduce the number of evaders in the county - in the past 12 months there were 1,800 in the OX postcode - and Oxford is in the top five per cent of worst offenders nationwide.

These days, the authority's methods of operation are sophisticated and high profile.

In the past, some people held the belief that the detection vans were a myth, established to create a culture of fear which caused everyone to pay up.

It was also thought that the vans swept anonymously around city streets, picking off victims at random.

But both these notions are dismissed by detection van manager Phil Carvill. He explained the methods now employed by the authority.

"We have a database of 26 million addresses and we know which have a licence and which don't. We even know which payment scheme people use.

"At certain times, we will target those without a licence with our computer system, which is called Lassy."

The computer set-up, fitted in the back of detector vans, works by picking up signals from oscillators in television sets. This is then shown on a display, which resembles a heart monitor.

The van parks outside the target's home so signals do not get confused with other houses and can then relay images from whatever terrestrial television channel is being watched.

It also indicates when a video, satellite or cable system is in use and must therefore have a television. Computers able to receive television pictures also need a licence.

Armed with this information, enforcement officers will take a statement from the offender and can also gain a search warrant from magistrates in the event of non-compliance. This includes ignoring the knock at the door.

The police can also be called in when repeated attempts at contact are in vane.

Residents receive numerous warnings before an enforcement officer calls but those who ignore them face a £1,000 fine and a trip to court as well as having to buy a licence.

The van itself has clear windows, aerials on the roof and Television Licensing written on the outside.

Mr Carvill says: "The sight of the van gives a subliminal message to people. It shows that we are about and they should get a licence.

"This does mean that we sometimes have problems, but it's rare and tends to be verbal abuse or people throwing bricks and stones at the vans."

For blocks of flats or other areas inaccessible to the van, a hand-held device is used, which works from a distance of up to 50 metres. It shows which direction the signal is coming from and how far away it is so investigators can stand outside the relevant flat.

Mrs Lodge says that there are different types of offenders.

"Some people think that they won't get caught while others, such as students in halls of residence, do not realise that they need a licence for their room.

"This also applies to shared houses. If you have separate rooms and separate tenancies then you have to have your own licence."

The only person exempt is the Queen and people aged 75 or over, who must still apply for a licence but do not have to pay.

Pubs, hotels, businesses, the BBC and even the detector vans themselves must have licences but many schools do not, along with people with small, battery operated sets.

Money raised from the licences, which runs into the billions, funds the BBC. The licence system is also employed by about 30 other countries.

Licences cost £109 a year for colour and £36.50 for black and white sets. If you don't have one, call 08705 763763 and prevent that knock at the door...