Songs like "Action", "Fox on the Run" (especally their own produced version on Desolation Boulevard), "Set Me Free", "Yesterday's Rain" and some other tracks are heavy metal to me. Slade's heavier well known-song from the 70's like "Cum on Feel the Noize", "Mama Weer All Crazee Now", "Look Wot You Dun" are more proto but like I sayed before, their later albums, like those in the early 80's, are better and heavier than many albums by other artists that was labeled NWOBHM.