Hi Sausagesupper, where did you get the key? If you used the same one left behind by the previous person then you need to get it changed.
Ring the electricity company and explain that you have received a bill. Tell them what key you used.
I used to work for npower and we had card meters (same thing instead of a key it was a plastic card.) You are meant to get a new customer card when you move it, ie you go to the shop, request a new customer card and put some cash on it.
The card contains a reference number and the info goes down the phone line from the terminal in the shop. You then put the key/card into the meter, the card tells the meter its reference number (called marrying the card/meter up) as it is a new customer card the meter then resets to zero wiping any credit or debit from the previous person.
Your meter then passes info onto the card/key, so meter number, readings, how much is on the meter etc. The next time you go back to the shop all this info goes down the phone line to the electric company and they now have the key/card reference number and the meter number so that any future money you put on goes against your account number.
This means you should get a bill every quarter for information only, telling you how much electric you have used and how much you have paid. So technically you should always have a little bit of credit on your bill.
That way, if the payments don't match the consumption the electric company can investigate why.
Hope this helps explain how the system works. Sometimes with Landlords, they will buy a new customer card but only top up the meter once meaning the details never get passed back to the terminal if the card isn't used again.