John, here is a V5C for an M20 showing a date of first registration as 1982. But at the bottom of the front cover it shows the date of manufacture (1942).
If yours don't have this information then I would do as you say and get dating letters and visit your local DVLA centre while you still can. Ron
Hello Ron
Thanks for your advice and suggestions will contact DVLA.
I found the following GOV UK site.
https://www.taxdisc.direct.gov.uk/EvlPortalApp/app/enquiry?execution=e1s1
Use this site to check out your vehicle Reg and make, it also tells you the year of manufacture of your vehicle held at DVLA.
I have printed a copy of the page and put it with my documents.
I will still ring them to see if year of manufacture needs to be on V5