I have tried to confirm this with FBHVC and DVLA but neither can answer the question and both simply advise me what I already know. I am reminded of my off topic post on flowcharts

When I think back to 1997, when Messr BLiar and Brown came to power, rolling tax exemption was frozen as at 31 12 1972. If the rolling process was the same then (Only based on 25 years), in April 1997, John Major's government would have passed the law, covering 1972 vehicles, which took effect in April 1998. In 1998, the new government didn't extend this.
I don't wish to start a political debate but based on dealings I had with the government between 1998 and about 2007, it was clear that the change in policy resulted because someone (Probably Gordon Brown) simply didn't approve of historic vehicles (If you unpicked and disproved their reasons, rather than give way, they concocted another, which you then had to unpick). I have a feeling that Mrs May might not last out the year as Prime Minister and since I have a 1979 car, I was just trying to establish when the law, bringing it in from the cold, will be passed (Should whoever takes over also dislike historic vehicles). It's not simply about the issue of paying road tax. Officially my 1979 MG Midget is still an old banger and when it becomes "Historic", there are some legal benefits. For example (Not that it will affect me) it would not be subject to the London pollution charges.