One all-inclusive price for parts and labour without surprises for maximum visibility: Get the condition of your lights checked before any work is performed and have them refurbished by Citroën expert technicians.
If your headlights are less bright than before, the headlight glass is dull, matt, or even completely opaque, or the headlights have become yellowed, it's time to have them refurbished
The material of the headlight also makes a difference: while the headlights of older cars are made of glass, the material used for almost all modern car headlights is polycarbonate. This wears out on contact with the elements and ends up losing its properties, especially on the surface. This is why the headlight is duller.
If the problem is inside the headlight, go directly to your Citroën service centre to get it replaced. Think about it when you get your WoF done too.
If the problem is on the surface, seek advice from your Citroën Authorised Repairer to see if it can be refurbished.