Fresh from a night's sleep, I approached the recalcitrant bolt again. In three seconds, I had the answer. The bolt was set down in a well, which prevented me getting the Allen key into the socket far enough. The only (very limited) angle where I could get the key in fully was blocked by the frame. So, looking at the situation from a different angle, it was the frame that was the problem, not the well. And when I looked at the frame to see what, exactly, was stopping the Allen key swinging, I found it was a thin metal strip with formed a guide to the horn wires. I bent it out of the way, and undid the bolt.
Obvious when you think about it. Not obvious at all when you are feeling 'stuck'. Although daylight and fresh head might have helped, I suppose.
The cylinder head is held on by six bolts. Yes, it is six - I checked with the manual. Five are easy to get at, but the sixth isn't. It's tucked away, underneath the front of the engine, and again the frame is stopping any kind of tool access. I may have to lift the engine a couple of inches to get at it.
One step forward, two steps back. So what's new?
Six, no, seven bolts . There was a useless little Allen bolt tucked away and apparently serving no purpose, but this time I spotted it immediately and gave it what for. The sixth bolt came out without a problem, strangely.
The cylinder head is now off. It all needs a good decoke, but it all seems OK in there. I am now abandoning the subtle tools in favour of a hammer and cold chisel to get the rusty exhaust headers out. Wish me luck.
 Nobody expects, ect.