Just to add a brief note on this subject. Be very very wary of so called energy company salesmen/women knocking on your door and asking to see your energy bill. I've had them several times from electrical companies (supposedly). Never ever show them your bill. What they are trying to do is take a particular number which refers to your particular meter and then without you knowing and authorisation switching your meter reading to their own. I've had to be rude myself to many on the doorstep who won't leave and have actually threatened them. The worst I have found is Talk Talk ( not an energy company I know ) who just don't understand the meaning of No. 

There are so many unscrupulous individuals out there who can come across quite plausible even producing ID, and never ever let them in your home to check the meter because you can't /won't produce a bill. If in doubt especially if you can't get rid of them, call the police.

Elaine