The procedure in my post worked for me. I'm not too sure an impact driver is as good with countersunk screws.
To elaborate, your assistant holds a plank of wood with the bottom end being restrained by their boot. Further up, the plank presses on the screwdriver and makes sure it doesn't slip, while leaning on the upper end. So the screwdriver is held in the slot by considerable force. You then turn the screwdriver with a pipe wrench, or if you have one that fits, a large adjustable spanner. Mine is over 18" long, cost a small fortune - it's a Bahco, but is a dead handy thing and the only thing I have that fits the hubnuts on the three-wheeler.
