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http://www.direct.gov.uk/en/index.htm

If you see a vehicle on the public road (car parks, driveways or housing association parking spaces don’t apply) with an out of date tax disc, you can report it online, by telephone or by post. You don’t have to give your name. Your report will be investigated and enforcement action taken, if needed.
What you need to report
Provide the details about the vehicle:

registration mark
make, model and colour
Provide details about the location of the vehicle:

eg outside 24 Anystreet, Anytown, Any Postcode)
any feature landmark eg opposite telephone box
If possible, provide:

the time the vehicle can be seen on the public road
name and address of the owner
Report the vehicle
Use the information you have gathered to report the untaxed vehicle:

Online

Use the Driver and Vehicle Licensing Agency’s (DVLA) online service.

Report an untaxed vehicle online (opens new window) By phone

Telephone the national untaxed vehicles telephone hotline on freephone 0800 0325 202.

By post

Send a letter to the enforcement section at your nearest DVLA local office.

Find your nearest DVLA local office What happens next
An enforcement field officer, who will make a formal sighting of the vehicle and complete a witness statement, will investigate the information you provide and take the appropriate action if needed.
Did you know?
The work of the police and DVLA against vehicle tax dodgers makes a major contribution to road safety. Around a third of motorists reported for not having vehicle tax are committing other offences - including no MOT certificate, no insurance and defective tyres.