BBC detector vans

There is a lot of discussion on the web about whether BBC detector vans actually work or whether they are a device to scare the poor and sometimes feckless. There is also a fair bit of discussion about whether the BBC actually take out search warrants to enter premises or not. Whilst digging around on the BBC Freedom of Information website, I came across this non answer to a question about the number of search warrants granted on the basis of TV licence evasion in 2006 (actually Licence was spelled "liscense" and the spelt "he"! The answer was "It is a criminal offence to install or receive......I can confirm that we do hold the information that you have requested, however, I am withholding this information ....prejudice the prevention or detection of crime and collection of the licence fee."

The same or very similar answer is given to other questions:
"How many TV licencing 'detector vans' are there?"
"On any given day, roughly how many vehicles are operating (ie trying to detect actaul licence fee evasion?"
"How many fines issued for licence fee evasion are as a result of detector van technology?"

Hmmm, what do you think that means?

There are some detailed answers on the technology and costs of detector vans so I assume that they do exist. Some more digging is called for.

For the avoidance of doubt, myself and Mrs NotaSheep do have an up to date TV licence, but we do resent paying for New Labour propaganda, digital channels that hardly anyone watches and Jonathan Ross's salary.