They are indeed Aluminium - Thicker than Bombardiers efforts though, while the bogies are cast steel.
But whilst a Desiro may be heavy, a Electrostar with transformer is even heavier. And the new 387s will have a larger battery pack as a result of the Harwich line trials. So an extra 5 or so tonnes heaver still.
Are you saying that the normal 387s (as delivered to Thameslink) are effectively carrying the battery weight of the trials 379?