Not hugely dangerous, as long as you turn the circuit off first! But the tricky bit is attaching the thing to the ceiling. Oh, and trying to join up the wires while balancing the weight of the fitting with your other hand and dropping the screwdriver on the floor halfway through, because you normally have to join up the wires before you can put it onto the ceiling, so you tend to run out of hands.
It depends how much of a big deal £100 is to you, I suppose. But it's a bit awkward fitting them and I don't think asking £100 for the job is at all unreasonable. My DH does them in our house, and there's a fair amount of swearing and shouting involved, and that's after he's done quite a few of them over the years.