I haven't had an electricity bill for a 18 months, and suddenly they want me to pay �750 straight away - what rights do I have?

If you have signed up to get your electricity from British Gas or another supplier, then you have to pay for what you have used. And there is no time limit on when your supplier can ask for payment.

The billing chaos will probably be investigated by industry regulator Ofgem, and as a result British Gas says there will be discounts for those customers that have waited longest and owe the most.

However, when pressed for specifics, all that British Gas was willing to say is that only "extreme cases" will receive money off.

The size of the discount will be decided on a case-by-case basis.

And consumers should not hope for too much help.

BBC News Online reader Mark Pittam was offered only �10 off a �1,000 bill after British Gas failed to send him a bill for three years.

If the delay is not your fault - i.e. you have tried to pay your bill but your supplier never sent you the bill - try to negotiate an agreement that payments can be spread out over a longer period.

What if I am unhappy with the discount offered?

Industry watchdog Energywatch says that customers waiting for a bill or facing hefty backdated charges should complain to their electricity or gas supplier in the first place.

If you are not satisfied with the response, you can ask Energywatch to take up the case.

Once a case is referred to Energywatch, supplier usually will not disconnect you because of your debts.

Energywatch says that suppliers take cases referred to them very seriously indeed.

"It is a sad fact but we seem to get a lot more co-operation from the supplier than the customers themselves," an Energywatch spokesperson told BBC News Online.

The reason is obvious: Energywatch can complain directly to industry regulator Ofgem, which previously has imposed fines on wayward firms and in once case even barred an energy supplier from taking on new customers.

Energywatch can be contacted on 0845 906 0708 or e-mail: [email protected]

My mother transferred to a new energy supplier half a year ago and is still waiting for her bill, what can she do?

The advice from Energywatch is not to ignore the problem - the onus is on the customer to make the supplier aware that you are still waiting for a bill.

Do not forget about it as some day the supplier will catch up with you and you will be hit with a hefty bill - some consumers have been presented with bills for four or five years worth of electricity in one go.

To be on the safe side, you should keep a log of all communications with your supplier and confirm telephone conversations in writing. Just because your energy provider is not efficient does not mean you should be likewise.

If you fear the eventual bill could be huge, then keep a record of the meter reading and ask for the standard electricity unit charge - work out what you are likely to owe and put it aside.

This will be a safety cushion for the day the bill arrives, and will be even sweeter if you can negotiate a discount.

Is it only British Gas customers that are being hit?

British Gas says the failure to bill customers that have switched suppliers is an industry problem, and indeed BBC News Online readers have reported similar problems with many other suppliers.

However, Energywatch says that it has singled out British Gas because the supplier "has gone hell for leather to sign up new customers without having the proper back up systems in place".

Ok, I'm not deterred, I want to change suppliers, what should I watch out for?

Check out prices and service records. There are a number of free price comparison services on the internet which have been approved by Energywatch.

Simply give them details of your energy consumption, and they will tell you who is the cheapest provider.

The Energywatch web site offers detailed price comparisons as well, which are regularly updated.