BMW E46 Xenon not working

I have a BMW E46 320D Sport 2003 Facelift.
A few years ago I bought a set of OEM Xenon headlights to replace the stock headlights (non-xenon) and fitted them myself.
Last night, I got a warning on the dash saying the left side (passenger) headlight was not working. Turned the lights on and off and the car on and off, no difference.
Checked the lights and the lefthand side was not working but the righthand side (drivers) was working.
I changed the bulbs around to make sure it wasn't a faulty bulb, but it worked in the drivers side headlight, so both bulbs work fine.

I have been doing some research and it seems to be a faulty ballast or ignitor? Is there a way to check which one and a place to get my parts fixed or a replacement part at non BMW dealership prices.
Would buying a second hand xenon headlight for parts be cheaper than buying a ballast/ignitor be cheaper?

Thanks guys