Not too much point checking the breakers yards as its a fairly common problem.
The Ballast is a small box of tricks that lives in the bottom of each headlight unit. The only way to get access is to remove each headlight. This in turn means at least dropping the front bumper to get access to the light fixings.
In my own case, I was able to remove the light without the bumper as it has had crash damage in that corner from a previous owner.
Its common for the headlight unit to allow water into the ballast which, being electronic is not to keen on h2o. I tried drying mine out but after a few days realised that replacement was needed.
I opted for a universal ballast of of the bay, £22 for 2 (just in case the other goes the same way) with adapter connectors at about a tenner each.
The result was a headlight which works great and cost about £20 to fix. Just went through an MOT last week so I know everythings safe. While I was at it I fitted 2 new bulbs of
too. In total I spent £60 and have a spare ballast sitting in the garage.
Hope you get yours sorted as the xenon lights are kak at dusk, even worse when only one works.