Note that in this months Todays railways 158 790 has moved from NL to Heaton to join the 16 Perkins engined ones 158 815-17, 842-45, 48-51, 53, 55, 59-61 which means Heaton is all 158 and 156 ?-?.
Although Northern allocations is not always an indication of where units are based my best guess...