I think this is interesting as a case study, so you might find it useful in case you're ever in a similar situation. It's not too gory, but there are some details about blood, so feel free to skip it.
So, I approached the police, and said that I was in St John Ambulance; they were happy for me to help out. Basically, they'd been called to the scene and found that the guy had already collapsed. He'd been drinking, but he'd also hit his head on the ground, so there was blood there. This is a tricky situation, because it's easy to make the wrong diagnosis; if he's incoherent and having trouble putting his shoes on, is that just because he's drunk, or does he have a serious head injury? Adding to the fun, he was Polish, and spoke very little English, which made it difficult for me to gather information. The police had already called the LAS, so I just had to keep an eye on him until they arrived.
I always carry a facemask and blue gloves in my rucksack; in this case, the police gave me a spare set of their gloves, so I didn't have to use my own supply. However, I didn't have any dressings with me, so I think I'll need to buy some (they're only about 50p each from St John Supplies). One common theme in our training is that when we find a casualty we should start by putting them in the right position and applying pressure if necessary; anything else (involving equipment) comes later. That's useful advice in a situation like this, where all I really had were my hands.
I checked the back of his head, and I could see him bleeding from a cut that was about 3-4cm long. There wasn't much blood, particularly given that this was a scalp wound, but I didn't want him to rest his head against the ground/wall; for one thing, that would increase the risk of infection. I didn't need to apply pressure, so I focussed on position: "If it's the head, put them to bed." In other words, the casualty should be lying down, with the head and shoulders slightly raised.
How do you raise the head and shoulders? It depends on what's available. When we're training, we often tip a chair over, so that the front of the seat and the top of the backrest are touching the ground (legs sticking in the air at a 45° angle). That means that the casualty can lean against the backrest, although they're using it upside down and from the wrong side. If you don't have a chair handy (which I didn't), another approach is to kneel down behind the casualty, so that their head and shoulders are in your lap. I've done that several times in a classroom, and it's easy: the floor is covered in carpet, so it's nice and comfortable, and I only have to hold the position for a minute or so until the instructor/assessor has seen me doing it. However, doing it on concrete is a bit less comfortable, particularly when I had to hold my position for about 20 minutes! At one point he half sat up, so I took the opportunity to shuffle round a bit, then I was in the new position for another 20 minutes. When I finally stood up, my calves were really aching.
The related problem was that I didn't have quite enough space to get between him and the wall, so I had my arm around his shoulders (to stop his head banging against anything) and my left leg supporting his back. He wasn't entirely co-operative, and although I had gloves on I still wound up with quite a lot of blood on the sleeve of my shirt, some of which soaked through to my arm underneath. Fortunately my shirt is red, so it wasn't too obvious, but I'll need to give it a proper wash. I've seen lots of eco-adverts saying that you should turn your washing machine down to 30°; on the other hand, you need to wash clothes at 60° if you're worried about infection from bodily fluids. Unfortunately, the label on my shirt just says 50°, since it's not intended for ambulance work. Bah.
Normally if someone had a fall like this, I'd ask them whether they remembered what had happened; it's not because I want to know, but I need to know whether they're suffering from short-term amnesia. However, the language barrier prevented this. His eye movement seemed normal (i.e. both eyes moving together), and I borrowed a torch from the police to check his pupil response, but he kept turning his head away whenever I tried that, so I couldn't get any results. Similarly, he swatted my hand away whenever I tried to check his pulse. When he quietened down a bit, I decided to check his breathing rate, but that threw up a new problem: I wear my watch on my left wrist, which was now on the far side of his neck, so I couldn't see it. In a case like this, I can see the benefit of the upside-down watches that nurses traditionally wore. I improvised by asking the police to time one minute (saying start/stop) while I counted his breaths. He had 23, which is a little bit high (standard rate is 16-18/minute), but not so high that I was particularly worried. Ultimately, I figured that as long as he was conscious and breathing, all I could do was monitor him until the LAS arrived.
When the ambulance arrived, the crew basically agreed with our assessment (i.e. that he was just drunk), but they took him off to hospital to be on the safe side; it amused me that he was going to the hospital where I'm based. One thing that impressed me is that they knew a few words of Polish, so they were able to communicate a bit better than I could; if I'm going to do any support work for the LAS in the future then it might be useful for me to learn the basics of some Eastern European languages.
Anyway, this wasn't quite what I intended to do tonight, and it delayed my homeward journey by an hour, but ultimately I'm glad that I turned back to help.