NeilStevens

Thanks everyone for the encouragement and tips. Awaiting delivery of a new socket to apply more force without risk of rounding nut, along with more lube. Though pretty sure it will not be overly rusted, the bike had barely been on the road for the first 10 years of its life.
But intrigued by Jambo's response - If it is safe without the alignment bolt then I may well leave it. I had kind of assumed it was more "structural" , or a least a fail-safe if the clamp loosened for any reason. I dont actually need to change the clip-on the damage to the bar was limited to the bar-end bolt bending and this little bolt shearing.
(The rest of the work is replacing the destroyed nose, headlight, front fairing frame, split in half clocks and repairing the splits in every other fairing panel!)