Ahh, Adrian may have a point. I had assumed the buffers were self contained and fully assembled with CH screw in place, before being pushed into the buffer beams from the outside face. (And not yet permanently affixed). I had also assumed the CH screw wasn't actually fixing the buffer assembly in place, but securing the sprung buffer head within its body. If however, for whatever reason, the CH screw needs to be inserted from the rear with the buffer already in situ, then the brake assembly is indeed in the way.