I hope this is useful for some and it may in fact already be known so sorry if it has but.....
I posted a few days ago how my factory fitted Xenons had gone out, well one had anyway, turns out the ballast had been soaking in water for the better part of a couple of years judging by the level of corrosion.
Audi wanted roughly Ł300 for a new one and the ones on eBay were still silly prices for used ones, (Ł120 northwards).
Cue the useful info part.
In a 'Rain Man' moment I remembered that when I worked at an Audi and Chrysler dealer in NZ when I was going through Uni, I had noted that the US cars used the same brand of some electric components as the Audis.
A bit of research later hey presto, Jaguars and Chryslers of the same era use the same Valeo HID headlamp ballast.
One Chrysler ballast replacement later I can see in the dark again.
One of lifes little wins.
Hope thats worth knowing!