[NTECUK]

If it turns when not engaged with engine it not pulling much current.
So any resistance in the circuit has less of an effect.
That's were the meter good. It shows you were the volts drop.
So if needs to be checked in the engine.
The biggest volt drop is across the starter (11 Volts probly as your loose some across the solenoid and all the connections ).
So check out the volts drop. Volts on your meter. Probly range will be 20volts. Put the -ve or common on the meter probe on the -ve battery terminal.Then the pos probe to the frame. Then keep the -ve battery terminal connected. Them +ve probe to the engine.
Any more than 0.65 volts is bad.
If that's OK with the meter -ve still on the -ve on battery -ve terminal and the positive probe on the input of the starter. The meter reading should be 10 volts min when you crank it.