I agree with Mitch.
I think more than 1 amp is not good for the Hornby motor and might even heat it up such that the windings are cooked. Given it more Volts will not help. I have a 6 Volt version and it takes less than 1 amp. Since you have to give it a push sometimes this could mean that one of the rotor windings is gone which could be checked with an Ohm meter.
Regards
Fred