If you are with one of the 'Big Six' on a standard tariff or on a prepay meter you are in for a nice surprise if you use one of the comparison websites to find a better deal. It shocks me how many years some people have been paying £100's extra per year to heat their homes. A web site I have used with great success is uswitch.com.
I am with Octopus Energy and have been for a few years. If you signup with this link-
both you and I get £50. Bonus!