About 3 years ago I used an online Gas and Electric switching service to move from Southern Electric and British Gas - the default local suppliers which is the generally the most expensive option - to a dual fuel scheme from Powergen (now called EON). I'm sure it saved money but I forget how much. I got £20 cashback by switching using a link from Mutual Points
So time's moved on and there's more money to be saved, and cashback to be reaped.
The best place to start is with Martin Lewis' excellent site, MoneySavingExpert.com, specifically the switching article.
I used a recent bill and today's meter reading to work out how much gas and electric I've used since December to give me an idea of my annual use (about 14600 kWh gas and 6800 kWh electric).
I then used MoneySuperMarket to get started. My current estimated annual bill is £1084.
Switching to a dual fuel option with British Gas would save me about £68.50/year (6.3%) and get me £30 cashback.
Switching just the Electric to the same British Gas option would reduce that part of my bill by £62.60 to £602.73 and get me £17.50 (or £20 according to QuidCo - UPDATE It has tracked as £20 so hopefully that will be the pay out in a few months).
Switching just the Gas to Atlantic would reduce that part of my bill by £23.50 to £395.33 but not get me any cashback (other providers had cashback but not Atlantic).
So dual fuel is an annual bill of £1,015.61 with £30 cashback
Gas and Electric sourced separately is an annual bill of £998 with £17.50 cashback.
So doing thing separately is a little cheaper, but I'd really like 2 lots of cashback. So I put my Gas details in to EnergyHelpLine.com who seem to offer £15 cashback for all providers.
So I've switched using MoneySuperMarket for my electric and EnergyHelpLine for my gas and should net an annual bill of £998 and get £35 cashback.