I saw this instore at B&Q yesterday and seems to be much cheaper than anywhere else.
It connects to your electricity supply at the fusebox and gives a constant readout on a remote display of how much electricty you are using in the whole house.
The theory is that by being aware of your consumption, you will start to switch things off and save money.
I'm still undecided though on whether to get one of these, are a simple plug in monitor that measures only one socket, also instore at B&Q for £9.98.
*please note that the link is to the manufacturer's website for information only*