Date of manufacture, not date of registration - here or abroad.

http://www.direct.gov.uk/en/Motoring...ses/DG_4022042 says:

Old vehicles constructed before 1 January 1973 (historic vehicles)
If your vehicle was constructed before 1 January 1973, you could tax it in the 'historic vehicle' tax class. Further information can be found on leaflet INF34.

• Download 'Exemption from vehicle excise duty for historic vehicles' (INF34) (PDF, 338K)

Also says:
How to tax your vehicle in the exempt tax class

You can change to the exempt tax class at your nearest Driver and Vehicle Licensing Agency (DVLA) local office.

Renewing your ‘free’ tax disc

DVLA will send you a V11 reminder to renew your tax each year.