Headtorch
The new small and light LED type headtorches have allowed significant weight savings for alpinists in recent years, though don't go too small (e.g. Petzl E-Lite, Black Diamond Ion), as these models aren't powerful enough for climbing in the dark. One set of new batteries will easily be enough to last the whole week so spares are unnecessary.You will also need to ensure that your headtorch can be fitted to a helmet.
Suitable models: Petzl Tikka or Zipka, Black Diamond Wiz or any of many similar models.
Quantity: 1