I suspect the 1348 date might be correct, as there was a protracted battle with his wife to have some of the dismembered remains returned after removal from public display. Isabella and Mortimer wouldn't allow such a monument, and his returned remains were housed in a temporary coffin. Only after Edward III had taken power from his mother and her lover would the spirit of reconciliation allow the monument to be built.
ferrum ferro acuitur et homo exacuit faciem amici sui
I have seen a number of these gauntlets but the effigy ones are there first I have seen with numerous lames along the cuff. All others have had splints like the glass ones or wisby gauntlets.
I'd say the 1349 dating is spot on. The shape of the bascinet, the vervelles, the arm harness and the gauntlets all fit with contemporary effigies and miniatures from England.