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Maybe you doing something wrong Ed?
We don't have access to an impact driver at work, drilling is the easy part, I use a smaller drill than the one Brompton provide then use the Brompton one to finish it off (quicker with less risk), the hard part is removing the bushes.
Don't think you ALWAYS have to drill out the bolts.
The Brompton kit come with a drill, if you're lucky you can removed one of the bolt meaning only one bolt to drill, however most of the time both bolt are siezed.
I kept the allen keys from a previous repair to try and removed the bolt (while holding the other one with an old Brompton allen keys).
Ben at Kinetics, who does A LOT of rear triangle swaps says that 99% of the time an impact driver does the job. Maybe you doing something wrong Ed?