Octavia II 1.6tdi auto non start

Hi Guys, got an Octavia II 1.6tdi auto, non start. Came in Customer stated starter motor failure, we replaced and still same fault.  No live to trigger/red starter wire. Checked all fuses , relays. checked ECU dry so no water ingress causing this. New ignition switch and brake light switch. It's reading keys on scanner. Only codes are low battery voltage, so got a jump pack on.We can start it with wire straight to battery from starter. There's no red immobiliser light on dash, do these suffer with cluster failure? Checked all wiring on starter system but all looking good so far. No4 injector is duff but don't think that will cause non start issue. Customer is reluctant to change that if we can't sort this fault 1st. Appreciate any help as we don't work on many Skoda's or in depth electrics. Thanks