I, until recently worked in this industry for a number of and wish to impart my experience for the help of people who need help to what sometimes seem to be very difficult problems encountered with energy billing.
As has been said before, when talking to the company it is in your best interests to keep calm and be resonable to the agent your speaking to as they are human and do genuinely want to help. It can also be the case that the agent is unsure of your problem in some cases as of what is said so try to keep the explanation consice because in reality the problem is usually fairly simple to resolve.
High Bill?
This is a wide ranging topic so i'll try to keep it brief. You need to ask a few questions to yourself first before disputing the bill, are the meter readings correct that are stated on the bill? If yes, then are the previous meter readings estimated, if they are the chances are is that the previous bill was underestimated and you are catching up the usage you had used. Unfortunately this is not the fault of the supplier, as you could of provided a reading anytime for them to rebill you (which they are more than happy to do). If the meter readings are incorrect, take a meter read and give this to the supplier, it is also worth noting the meter serial number which is found on the face of the meter (Gas usually starts G4- or a year of manufacture or just a string of numbers, Elec usually starts with a letter then 2 numbers and a letter again) check all these details against your bill. If any are incorrect then inform the supplier.
Gas usage changes dramatically from summer to winter so don't be surprised when your winter bill is many multiples of your summer bill. Electric usage is fairly constant (but does change) through the year unless you have storage heaters and are on Economy 7.
As a rule, your meter is not faulty at all. I have seen many people try and believe it is their meter registering incorrectly and it isn't. Honestly. It also costs you to have it tested and if it comes back that it is regestering correctly, you will have to pay for the test from your own pocket, as the jobs are not done by the suppliers and rely on a independent company to test. Only have the accuracy test if the usage (KwH,not the bill amount) are significantly different from similar periods of the year in previous years. If this is the case ask your self has anything changed from last year? Perhaps a redundancy, a new baby, an extension of the house, new appliances (Plasma TV's!) etc do have a large effect on the usage.
Transfer of supplier problems
Are you getting billed from two companies for the same period of time? You need to check the MPAN (Electricity) or MPRN (Gas) (Supply No's to you and me) to make sure they are the same. These are found on your bills, if they are different, chances are that the new supplier made a mistake in selecting the supply number to take over (this is an all to often occurance, due to addresses showing on the inter-shipper databases being either very similar or very vague). Give them your Meter Serial Number and they can match the supply number to this to get the correct supply number and they will correct this.
No Bills Received?
Have you been trying to get a bill but no supplier can tell you who supplies? This is quite common with new build (Up to around 5-6 years old) properties. These supplies are typically not administered by Transco (the most common) which makes them difficult for any supplier to find. The supplies are administered by what is known in the industry as an IGT (Independent Gas Transporter) and you need to give the Meter Serial Number (typically starting with a year e.g. 2007 then a six digit number) give this to the supplier (ask for the escalations department, as they have access to the IGT databases whereas your front-line agent may not).
Estimated Bills
Suppliers will estimate bills when no valid meter read was recieved. If you know your meter has been read, possibly many times and your supplier continues to estimate, this suggests they are recieving the meter read but is not valid. This can happen for many reasons but the most common are is that a meter exchange has occured but the supplier wasn't informed (possibly for an emergency job). Check the Meter Serial Number on your bill and see if it matches with the on your meter, if it doesn't, write the on the meter down carefully. There also should be a sticker or tag on or around the meter. This usually gives the exchange details, copy down all the details on there, there should be the date of exchange, Off Index (final reading of the old meter), New Index (the reading the new meter started on). These are essential details for accurate billing.
Sometimes if you've moved in or transferred supplier and keep receiving estimated bills this could be because the reading you started on is higher than the one on your meter currently and this will stop accurate bills going out. Inform the supplier of this and they can change it for you so accurate billing can take place.
Estimated bills are your responsiblity to give correct meter readings to the supplier as noted at the top of the page under High Bills. The supplier has only to read gas meters legally every 2 years, everything thing else the onus is on the customer to make sure the readings are correct i'm afraid. If they are estimated due to no access, perhaps because you work this again is not their fault i'm afraid and is not a valid reason for complaint as you can provide readings at any time of day, online, on the automated phone systems or a simple phone call to customer services.
Moving Home?
It is of your utmost prorities to inform the supplier of your moving in or out of a property. Give and RETAIN the accurate readings you took during your move (i recommend a Hi-Res photo of your meters on the day of the move, clearly showing the date and read). Failure to inform them of a move and not receive a bill for a long time and then get a large one in one go for the time you've lived there means that you are not entitled for protection under the back-billing over 12 months rule (I'll get to that in a bit) as it is your fault that you didn't provide the details and are liable for the entire energy usage at that property during you living there.
Have received a new final bill which you already paid? This can be caused by the new tenant disputing the meter read of them moving in which can cause a new final bill to be sent for your account. In this instance if you have your photo's email them to customer services along with a consice explanation, do let them know via phone as well to make sure. Unfortunately, instance your dispute isn't really with the supplier if you have a problem, it is with the meter read that the new tenant provided. Your supplier should help and rectify the problem by putting the meter read back to how it was. If this happens again however, it becomes a 3rd party dispute between you and the previous tenant. Please contact them, agree the reading (this is assuming you haven't taken photo's) then inform the supplier of the agreed reading.
Back Billing
Ah, one of the most quoted things on these type of forum! I'll give you the gist of it. You are protected by the ERA code of billing if: You have received a bill for more than one year when you have never received one before. To clarify, this means that you have taken resonable steps to inform the supplier that you have moved in, in that period of time. Failure to do this means that it is your fault and you are liable for the energy used at the property during the time you lived there. If you have taken resonable steps and have recieved a bill for more than 12 months usage but have not received one before then they can only charge you for 12 months usage, nothing more.
If you have been receiving bills before and have received an amended bill that goes over 12 months you are not entitled to the back billing rule unless you have had twisted meter reads (Economy 7) and the supplier has recieved the correct details from the industry but had it incorrect on their billing system.
To summarise that last paragraph, unless the supplier has been given the correct information by the customer or industry and has continued to bill incorrectly for more than 12 months, then the customer has to pay the bill in full.
The summary in full is found http://www.energy-retail.org.uk/documents/BillingCodeFAQs15Oct08.pdf.
That is all for the moment, there are many more scenarios but they are the common ones which are not always explained (i find) on internet forums.
If you wish for anymore information, don't hesitate to ask.
Regards.