That's really strange. I was with EON and spoke to a nice M&S man at one of their stores and he said he could beat EON by about £80 per year.
I went home and rang M&S to get a firm quote and they said they couldn't match EON because I administered it online and had paperless billing.
I next tried uSwitch. M&S didn't even feature in their list BUT nPower beat EON by well over £100 a year!!!
Today my gas and electric started coming from nPower.
It's hard to believe but my lights are now brighter and the gas cooks twice as fast;-)