In reply to alexgoodey: Pencil and paper (surprising how many first aid kits don't have this, pretty essential imo), reflective tape on gear/back of helmet, spare map and compass, spare head torch, vaseline, extra blister stuff. I'm sure you've got these things already but always worth remembering the small things before you start getting techy.
Also, I'd put your own first aid kit together instead of relying on one already made up, then you can play with everything before using it. Having used the contents of two first aid kits this summer it's surprising how shit they are.
For e.g.:
-those thin weight saving rolls of tape are impossible to use with bloody hands, rip easily and aren't very sticky anyway, wtf, don't they try this stuff out first?
-The "petrol station" gloves they put in make it hard to do anything with your hands, useless.
-Some foil blankets in First aid kits are tiny, not that you need one if you have a bivvy/bothy bag or thermal blanket but it's worth getting everything out and seeing what it's like instead of assuming that the people who make the first aid kits put some thought into the usability, as apposed to the weight on the label.
If you're making your own kit you can buy the wee first aid pointer cards to put in, it doesn't hurt to have a memory aid to look at when you're stressed. Imo it's worth writing out the things you need to know/questions you'll need to have answers to before calling the MRT, and placing the list with that card. Name of casualty, age, visual description and number of group, your location, hazards, description of the injuries etc. When you're stressed it's much easier to just find the answers to all the things on your list rather than wasting time thinking "have I missed something". These are perhaps unnecessary if you're first aid trained or you work in a related area but for the rest of us it's all probably going to come as a shock so knowing what you have and just following instructions might make you calmer, quicker and safer.