NEXT time your electricity or gas bill comes thumping through the door on to the mat, you could be excused for not being too sure you can trust it.

Cases such as that of York shopkeeper Trevor Fenwick, who was incorrectly sent bills for a total of £30,000 in 18 months by his electricity company, Independent Energy, prove that mistakes can be made.

And if they can be made on such a grand scale, who says smaller mistakes can't be made too - mistakes that it would be less easy to spot?

After all, it's never easy keeping track of just how much power or gas you've used - and so of how much you legitimately owe.

Easy enough to keep track of the meter reading, true: but how do you convert that into money? And anyway, half the time we're not in when the electricity or gas man comes to check the meter, and so are sent an 'estimated' bill instead. How on earth do the supply companies know how much gas or electricity we've used if they haven't actually read the meter?

Ofgem, the gas and electricity regulator, says mistakes do occur - and admits some of the new companies bidding to supply electricity appear to be especially prone to mistaken billing.

Which is a shame: because if you do shop around, there are some real savings to be made.

Figures supplied by Ofgem itself reveal that if you live in the Yorkshire region, regularly spend more than £400 a year on your electricity and pay by direct debit, you could save over £40 a year (rates from April this year: new tariffs have recently been agreed) by switching your supplier: a saving of about ten per cent. If your bills are smaller, your savings will be smaller: but they're still there to be made. And there are also savings available if you switch gas supplier: up to £100 a year if your bills are large.

But is it worth the hassle? And if you do swap, will you end up being hit with a huge and patently unrealistic bill, like Mr Fenwick?

Ofgem says it's actually surprisingly easy to make the switch. There are over 20 companies now ready to supply your electricity - and a similar number waiting to offer you gas. Many offer dual gas and electricity accounts.

It sounds like a minefield: but Ofgem has factsheets detailing the kind of savings you could make each year by switching from one supplier to another. They're broken down by payment method, and by whether your annual bills are small, medium or large - making it easy to see at-a-glance how much you could save.

Simply call the Ofgem freephone helpline on 0800 887777 and say you're thinking about changing your supplier.

An Ofgem spokesman said that, despite the teething problems faced by some new suppliers - the electricity supply market, in particular, was only deregulated in May last year - Ofgem do recommend consumers to shop around.

"There is no penalty for switching, and it is quite a simple process," the spokesman said. "Simply inform both your new supplier and your old supplier, and let them do it."

But when you get that bill, how do you know you're not being overcharged?

Check, check, check, say Ofgem. Make sure your supplier lets you know the unit charge for your gas or electricity: try to be in when the meter man calls, and if not make sure you take your own meter reading and inform your supplier. Then check your bill when it comes.

If you think you have been overcharged, contact your supplier first. If you don't get satisfaction from them, try Ofgem - the same 0800 887777 freephone number - for electricity supply problems, or the Gas Consumer Council for gas supply difficulties, on 0645 060708.

If you are concerned about your telephone bills, it is worth asking your telephone company to itemise all calls. There will be no charge for this. Your telephone company is also obliged to provide you with up-to-date tariffs for all types of call. If you're unhappy about your bill, contact your telephone company first, and if you're still not satisfied, get in touch with Oftel, the telephone regulator. The number to call: 0845 7145000.