my understanding was that it was a return for the current flowing from the live wire?
Correct, the neutral cable goes back to the substation at the end of your street where it is connected to the ground/earth. The substation gives all the electrons in your cable going to your house (live) a big push, 230 volts worth in fact. They are shepherded through your house wiring in the copper and end up at the substation again where they end up connected to the ground. Earth is where there is no push on them at all, 0 volts, and is where they are all desparate to get to as soon as they can
if you touched the neutral side of a load under power would you get a shock?if so how can it be 0 volts?
Lets say you are standing on the ground with wet feet so connected to earth, your body is at zero volts.
In the cable in your house is an electron waiting to go go into a switch to a light, but the switch is turned off. This electron is being pushed hard by the substation at 230 volts and so has lots of energy, so much that it needs to be kept inside a wire surrounded by plastic so it doesn't escape. It wants to get rid of the big push, really wants to get to the ground which is at zero volts, but right now doesn't have a way to get there.
Now turn on the switch and the path to the ground is clear, except first it has to go through a very narrow wire in the light bulb. To get through this wire, the electron has to force its way through, heating up the wire so it glows bright white. And as the wire gets hotter, it gets harder still. As it goes through the light it loses more and more push, and so has less and less volts. By the time it gets to the other end of this narrow wire in the bulb all the push has gone, the electron is absolutely knackered and has lost almost all of its energy, it only has enough to get through the big fat neutral wire back to the substation. As this wire is very thick this doesn't need much push at all and so the electron in the neutral has very little push energy indeed.
Now lets say you were to touch the wire just before it went into the switch. Remember this electron at 230v has a lot of push behind it, and it finds it far easier to get to earth (which you are standing on) through your body than the thin wire in the light (or if the switch is not yet turned on via the plastic surrounding the copper cable). So through your body it goes, and as it forces all the electrons in your body which are in front of it to move, this energy is let out and you feel it as a nasty shock. If enough electrons get into you and start pushing hard then it could be enough energy to stop your heart or burn your skin.
Now lets say instead you touch the neutral cable just after the electron has come thorough the light bulb, remember the electron is knackered after having pushed through that narrow hot wire. It is no longer at 230 volts push, they all got used up in the light buld wire, it is pretty close to zero. It still wants to get back to earth, but is going to say it's a lot easier to get there via the big fat copper cable than it is going through your flesh and bone, so it will go back via the neutral. Now some will be bloody minded and decide to go to earth through you, but not many and they aren't pushing hard at all, and so little energy gets into your body that you won't even feel it and definitely not enough to do any damage.
