From link below:
Release of another vehicle’s keeper details

You can obtain the name and address of the registered keeper of another vehicle if you can demonstrate ‘reasonable cause’ for needing the information. Information on the release of information and circumstances that DVLA has previously considered met ‘reasonable cause’ is available on the link below.

Scroll about 1/2 way to where it says:
Enquiries by post : - Release of another vehicle’s keeper details

http://direct.gov.uk/Motoring/Owning...066&chk=4gj8df