To MikeK.
Thankyou for that reply which was comprehensive and appreciated. Yes I understand all you have said and now I have a further question for you.
Snyders sell an adapter to attach to starter motor and enables that to handle 12V. See the attached link.
https://www.snydersantiqueauto.com/modern-starter-drive
or is the "modern starter drive" what you are calling a "barrel drive"?
that would be simple and save me seeking professional auto electrical services and thus be much cheaper too. I will modify in accordance with your recommendation.
and thankyou again.