Power delivered by a plug Ok so i know that (in the UK) the voltage of a mains supply is 230v. But when we plug a item e.g. a tv in to it what determines how much power is drawn from this supply. I presume it is the current but what prevents all the current in the mains going to your device? Also what happens to the power that is not used?
 A: The current drawn is simply given by Ohm's law:
$$ I = \frac{V}{R} $$
where $R$ is the resistance (more precisely the impedance) of the TV. The impedance of the TV is down to the way it's designed, so the manufacturer will determine how much current, and therefore how much power, it draws from the mains.
I've simplified a bit because mains electricity is AC not DC, so the current will also be AC. In addition the impedance of the TV may not be completely resistive, in which case the current-voltage relationship is more complex than I've suggested. Still, the simple Ohm's law will give you a good idea of the current drawn.
Re your question:

Also what happens to the power that is not used?

The National Grid go to considerable trouble to ensure they are generating the same amount of power as all the grid users combined are using. Luckily power consumption is fairly predictable. They maintain reserve capacity to deal with unexpected jumps in power usage. If more power is being generated than is needed it's fairly easy to cut the power generated by coal or gas fired power stations. Nuclear power stations can't be ramped down as easily, and so the power isn't wasted when it's not needed various tricks are used to store it e.g. the hydraulic storage Glen mentions in his comment.
