Ahhh...Crecy. Sir Patrick, have you ever read the Archer Series, by Bernard Cornwell? Set squarely in the timeframe of Crecy, it details the rise of a young English archer in Edward III's army. A great read, Cornwell has a way of depicting the horrors of war in such a way as to be able to almost smell the filth and the blood, to almost see the bodies writhing in pain from being hammered, pierced, slashed or dismembered by all manner of weaponry. I went on a tangent again; apologies.
As for the Pembridge helm, we know that it didn't come after 1370; as you'd stated, TBP's helm is much like it, if more ornate. Probably safe to say that you could get away with wearing one if that was your desire.