Starter motor isuues

[Bengaku]

Hi guys bit of a strange one starter motor was not working on my 1971 MGB GT so I had a replacement hi torque ordered up replaced the starter motor and it's making a whirring noise now had a mechanic check this he seems to think it's a power issue the battery is fine but just to be sure had a battery I know is good and put jump leads on this still same issue checked the wires on starter motor they are on the correct way so if the starter motor is new and seems to work but doesnt seem to be getting enough power and the battery is ok and even with a booster pack it still wont start what else could it be I'm in uk scotland so this is a right hand drive car as I know some on here are from usa Canada etc thanks for looking regards Paul