Andy G, if the components are removed then it will only charge the battery, but not protect the battery from overcharging. It is not good for the battery to overcharge.
So, if you put the components, then when the battery is charged that means, when it reaches to a minimum voltage 6.8, current will start flowing through the zener diode ZD1. It will trigger the transistor - T1. And now almost all the current will drain through the transistor's collector and emitter. Not enough to charge the battery anymore.