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From the dvla site

THE REGISTRATION DOCUMENT/CERTIFICATE (V5/V5C)

Never buy a vehicle without a registration document/certificate even if the seller says it has been sent to the Driver and Vehicle Licensing Agency (DVLA) for changes.

Hold the document up to the light - the DVLA watermark should be contained within the layers of paper; satisfy yourself that a fraudulent watermark has not been merely printed on to the surface of the paper.

Remember that the person recorded on the document may not be the legal owner; it is not a document of title.

Satisfy yourself that the person selling the vehicle has the right to do so.