Some people are missing the point here.
DVLA use the date when a vehicle was first registered.
BUT the government applied Historic Status using the date of manufacture as a reference!
If a car was registered in the year following manufacture, but qualifies for Historic Status then the DVLA needs proof!
My car was registered in 1974, so became historic from April this year. If I had bothered to go down the heritage route to provide the DVLA of proof of date of manufacture then I could have changed the class to Historic last year. This would have meant I would have to send all the relevant documantation to Swansea and wait for them to do it. I Just taxed the car for 6 months instead.
In the 2015 Budget George Osborne confirmed that the rolling break would continue. Hopefully 1976 should escape any changes to government.
I didn't realise that the relevent data from Glasses Check book would suffice. Hopefully this will be of use to many, and is a far cheaper option than getting a heritage certificate.
If a vehicle qualifies automatically then it can be done at the post office. If proof is needed then it's Recorded Delivery to DVLA Swansea.