I also had a D14/4, 1968 vintage, was definitely 6 volt. Good advice to check the bulbs, as 12v conversions are now readily available.
Seem to remember that it chewed 2nd gear regularly. Also remember that you had to take the damn thing to bits to replace the kickstart return spring which busted at regular intervals.
Ahhhh, kickstarts, eh? Those were the days.............