Generally speaking it's a rolling 40 year exemption based on date of first registration for both tax and MOT when vehicle should also receive historic status.
Currently if your vehicle was first registered between 1st and 7th January 1983 then road tax will be zero although you still need to apply for it each year, when you will be asked to make a declaration that your vehicle has not been extensively modified during the last 30 years such that it qualifies for MOT exemption.
Vehicles first registered later in 1983 will have to wait until 1st April of the following year to be allowed exemption. Until this happens I would ensure MOT status overlaps any of these dates.
So that road tax renewals are all due on 1st January for my elidgible vehicles I retaxed them prematurely (at zero cost) to bring the reminders in line.
