Won't you be replacing bits if you buy a new set? why not just swap the ballasts see if thats the problem.
open bonnet
unplug working ballast
unplug non working ballast
plug working ballast into no working side
plug non working ballast into working side
switch on lights
if the light is working on the none working side, its the ballast if not then its something else.
i no i wouldnt want spend £30 on a kit and it turned out to be somthing else.