The findings were the latest update from the CMA's full-scale probe into the sector which is dominated by the Big Six firms - Centrica, SSE, npower, EDF, Scottish Power and E.ON.
It has updated a statement it published at an earlier stage of the investigation in July last year. The CMA said it had not yet reached any conclusions. Provisional findings will be published in May.
10 simple ways to keep your house warm this winter
Energy customers overpaying by £234 a year
Of course you should make sure the doors and windows are shut properly, but you should also check each one for draughts too. A good old fashioned draught-excluder will stop the wind whipping under the doors, and draught-proofing strips around doors and windows should see off the worst of the chills.
It's not the kind of DIY job that anyone loves, but a quarter of the heat in your home is lost through the roof, so it makes a big difference. Once you've insulated the loft, the roof space will be colder, so make sure you have insulated any water pipes and tanks, and draught-proofed the loft hatch.
If you have thicker curtains, ideally with a thermal lining, you're likely to lose 25% less heat through the window. It's also worth considering curtains over external doors, to prevent heat from escaping. However, make sure you draw them back during the day to make use of any glimmers of sunshine we get.
If you don't have a working chimney but you do have an open fire, then you'll be losing heat through the chimney. If you place a chimney balloon in the chimney and inflate it, it will trap the warm air in.
If you stick the sofa in front of the radiator you'll waste a fortune keeping the back of the sofa warm.
Many people will remember elderly relatives applying tinfoil in a hap-hazard manner years ago, but it doesn't have to be noticeable, and will reflect half the heat back into the room.
Bare, varnished floorboards have been popular for a while, but unless they are carefully filled and draught-proofed, you can lose 10% of your heat through the floor. If filling the floorboards is impractical, a carpet may be a simple solution.
There's no point in heating any rooms you don't use, so turn off radiators in unused rooms, and heat the rooms you tend to occupy instead. Once the spare room gets chilly, you'll need to keep the door closed, and use a draught-excluder to stop the chill spreading.
If your kitchen is the heart of the home you don't need the house so warm during dinner time, because you can use the warmth of the oven to keep you all toasty. Get some baked potatoes in the oven, some soup on the hob, and no-one will notice the rest of the house has grown a little cooler.
It may seem a bit Victorian, but having a woodburner in the fireplace allows you to burn a cheap fuel, and enjoy the heat without the smoke. Burning wood costs less than gas and two thirds less than electricity, so you can stay toasty for less.