rarely (make that never) take first aid kit but used to when taking kids underground, it included;
small knife,
roll of thin band aid wadding - covered in insulation tape to keep dry,
insulation tape to wrap round wadding on wound - plasters come off,
few wipes - just to get worst off cut if needed before covering it up,
matches - in a film case to keep dry - dont forget to have the stricker in there too,
clear survival bag - cut hole in end place over patient to keep warm,
candle - to put under survival bag - this really does warm people up well,
whistle,
mars bar,
thin ballaclava - more to do with kids getting cold than first aid,
length of sling - not totally neccessary,
glow stick - again not always useful but doesnt weigh much.
all fits easily into a small bdh and is ample suplies to either get a minor injury out or leave a serious one down in comfort while cave rescue arrive. never really needed it but have used the survival bag trick on a kid who got too cold once.