In reply to beardy mike:
When Calum and I did The Fish we took silver foil bivi bags and had the worst nights sleep of our lives. That said, we didn't die, so I guess they (just about) did the job.
In retrospect I would have taken a super-light sleeping bag and am sure it would have been 1000x better (and not much heavier either).
All of that aside, Mike's point is an accurate one - it could be roasting, it could be freezing. Plan accordingly.