Thanks for your reply. The circuit is a common two strand (live and earth) direct DC feed from 2 leisure batteries via a lighting master switch and 15 amp fuse. so I am not sure how easy it would be to install any sort of voltage regulator + heat sink. As you say without specifications you are blind, as am I, hence why I am busy interrogating various supply options to find a compatible "stalk" lamp that will work on the approx 14 volt input - which is proving quite hard!
It sounds perfectly normal - the nominal voltage of a car battery is actually 13.8V, so 14.29V isn't outrageous, and is well within the voltage variations on a car battery. It's highly unlikely that a small voltage increase would stop it working.
However, it's a little concerning that you describe the wiring as 'live and earth' - it's no such thing, that's a mains AC term (and is wrong even then - it would be live and neutral) - for a DC lamp the correct terms are positive and negative. Are you
absolutely sure you're connecting it the correct way round? - stick your meter on the lamp wires when it's connected, and make it reads positive on the positive wire (and negative on the negative wire). If it's connected the wrong way it won't work at all.
Do you have access to a variable power supply?, if so connect it to that, and try powering it from a power supply set to 14.29V and see if it works.
You might also try connecting it directly to the leisure battery, and see if it works then.
Edit: Just a quick thought?, how are you actually connecting them, is it some kind of plug and socket in the vehicle?, and is it two pin or three pin?.