To transfer the registration number, vehicles must be:

• currently taxed

However, if the vehicle that currently has the registration number is not taxed, you can still apply to transfer provided:

• there is no break between the date the tax disc runs out and the start of the SORN
• and the tax disc ran out no more than 12 months before the date you apply

http://www.direct.gov.uk/en/Motoring...tes/DG_4022576