Brian sculpted the armour and then the plasterers at Elstree made the moulds from this, then fibreglass castings were made to vacuum form over. These were then sent to AA after the vac machine at Elstree was faulty, so AA continued to pull the majority of the armour used in the movie.
It's believed AA did some refining to the helmet moulds in order to make them more user friendly, though this is not concrete, just what AA has said. It's been proven he did not sculpt the helmet or armour, but he may have sculpted the ear caps, knee caps and other greeblies to make up the complete stormtrooper as we see it on screen compared to what was made at Elstree before filming started.
Just a bit of background info there