Alistair, this is the perfect example:
I have been in this exact situation several times. I would have sounded my horn @ 33 seconds in this video (16:31:54), as well as slowing more on the approach to the corner. At this point the other car is out of sight, yet well under 50m away, easily able to hear a horn even in an insulated mercedes. It might seem that this is a little close, but since this collision occurs in under one second of the mercedes becoming visible, that extra two seconds or so triples the warning time that the other car gets, which makes the difference between coming to a stop facing each other and a collision. On every occasion the other car has heard the horn and is already slowing down before we see each other. We may have come to a halt on occasion just a few inches apart, but as yet no collision has occurred. Maybe this is why I don't have to make claims against other people...