In reply to beastofackworth:
I just take it everywhere, It's been used a few times too.
The most serious accidents I've been involved in have been sport climbing, and driving to a crag.
Remember the point is to save someones life, not to fix up a split tip.
But you want to make it light enough so you actually carry it.
In order of importance
Gloves (very important, you have filthy hands, they have horrible diseases)
One very large dressing (I think I have a no3 ambulance dressing)
Crepe bandage
Thick Zinc oxide tape
Saline solution to wash wounds / eyes ect...
Waterproofed paper and pencils (for keeping track of a casualty over time)
Tweesers
Small square of cling film (lightweight burn bandage)
(mine weighs about 180 gramms)
Also I keep some Plasters, as it looks a bit ridiculous if you mate has a graze that is gently bleeding on his new soft-shell, and the only option is a number 3 ambulance dressing
I used to have a lot more stuff, but took most of it out to keep weight down.
Also if I'm away from the road, I make sure the team has enough clothes to spend a stationary night on the hill, usually this means that I take a Blizard systems survival tube, if someone is injured then they can't move, and the biggest risk is hypothermia.
NB I am not qualified to advise you on first aid kits.