I wouldn't call it "very difficult", just moderately frustrating. I've done it many times without pulling the wires, by just sitting in the drivers seat with a towel on my lap to catch the bits I drop.
You will have to take the wires at least partially loose (8" or so of slack) and loosen the 3 "grub" screws in the steering wheel. That should be enough to let you pull the head back from the wheel far enough to undo the 3 flat head screws on the back (accessible through the 3 holes in the metal plate) and then the 3 round head screws that hold the face plate and horn button on.
Collect the face plate, horn button, spring, and distance piece. Then you'll have to hold the appropriate nuts (chain nose pliers will do if you're careful and don't have a 5BA wrench handy) while you remove the two screws that hold the horn contacts in place.
That should let you get in far enough to access the screw that holds the lever; without opening up the section where all the fiddly bits are.
I've stuck a few photos up at
https://s258.photobucket.com/albums/hh260/TR3driver/Control%20Head/ Someday Real Soon Now I might write an article to go with them, but I'll have to go back and retake the ones that show the little pivoting pushrods installed. The photos up there now show them installed backwards!