Well that was a struggle.
It turns out the retaining brackets were the least of the problems - poppers were in the wrong place. When the aero advert said 'holes drilled in error' I thought that meant the extra holes I could see not the holes the poppers were in. Or it could be that they're all slightly different or TADTS.
So having drilled the poppers out twice, fashioned a reinforcing plate to cover the now holey carbonfibre, fitted and adjusted the mirrors it was time to cook dinner so job for tomorrow is drive it!