Rapha, nice but a bit pricey?

As well as a jacket, if there is chance of poor conditions, I would take:

• armwarmers
  
• kneewarmers
  
• thin but warm full finger gloves (apparently some people take plastic disposable ones)
  
• thin but warm hat/buff.
  
• shoe covers of some sort, e.g the condom ones or even plastic bags you can put inside your shoes
  Put the lot in freezer bags and wrap rubber bands round them to pack them down.
  
  

@Señor_Bear: I think you are underestimating both how changeable and how serious bad weather can be on a mountain (or have a team car...). Went to Ventoux last year, 23 C variation in a single day (max 34 C, min 11 C). It doesn't take a lot of descending in the cold to really cause problems, and it's not a big deal to carry a few extra items.