I assembled every socket extension bar I have to the 1/2" socket and secured this in place first, so the naked self tapping bolt was protruding through the bracket in it's desired location.
Having 4 extension bars permitted my wife to be far enough out the way so that I could contort and lie across the nose of the car to just about force my hand past the extension bar, between it and headlight support bracket without removing too much skin, although some was sacrificed.
From there I was able to just about get enough fingers round one part of the horn to blindly flail it around over about 5 agonising minutes until it felt like the bracket hole had found the pointed end of the bolt. At this point I asked my wife to turn the extension bar end in the hope that the bracket was sufficiently well orientated for it to grab the threads without crossing them. Eventually it bit and we were able to nip it up.