It may sound a tad obvious but my passenger side Xenon headlight is not working and therefore the dashboard light is aluminated identifying that there is a fault. It's been playing up for a while...sometimes coming on, sometimes it doesn't. Is it likely that it just requires a new bulb or may it be something else such as the ballast unit could be faulty? I suppose i should buy a new bulb and take it from there? Any suggestions? How much are these bulbs generally?