This suggests you can go for MOT exemption after exactly 40 yrs, but Tax exemption must wait until the April of the following year.

https://www.fbhvc.co.uk/dvla-mot-questions-answered


"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10