For the benefit of people like me who only look at pictures, when following the instructions to change the top shock bearing for the new Polyurethane bush (wonderful piece of kit by the way).
There is a line of text that says "Remove Left-Hand subframe rail", don't ignore this because you will spend spend ages swearing at the airbox which will not come out.
Instead look at the picture below and undo the two bolts to remove said rail, I guarantee it will save you time and stop you feeling silly trying to figure out why you are the only person in the world that cannot remove the airbox.