for want of a slightly less proud-headed bolt

It's either the bolts of the zip-tie left over from the previous owner which prevented the new (slightly more compact and less recliney) bike seat from being attached to the rack after purchasing it from someone off the internet and it would have taken far too long to swap the fitted rack for the one he was selling with the seat so some stretching and sneaking and bungee was required to get it home, albeit with a great deal of rattling and going carefully round corners.