Accept the offer. Southern clearly believe the offer was fraudulent, and couldn't make it stick

If it's to your benefit to go back, then take it. Sounds like Southern accept that the offer was effectively fraudulent. In those circumstances, they coudn't make it stick, so they are not going to try.

I was approached in M&S by Southern Electricity to change my supply from British Gas I was offered a fixed tariff until January 2013. I was given my prices of dual fuel which I accepted and signed a contract. When I got my first bills I was charged a dearer rate. I contacted Southern Electricity and was told the salesman had quoted me the wrong figures. Therefore they are willing to send me back to British Gas as though I had never left. Surely if I have a signed contract do they have to stick to it. Please give me your view.