 The Midcounties Co-op picked a good day to announce plans to enter the home energy market. British Gas owner Centrica's warning about soaring wholesale gas and electricity prices will encourage even the most passive of households to consider shopping around before winter comes round again.  If it does what it promises to, the cuddly mutual will get a warm reception from many domestic energy users fed up with the confusing tariffs, lock-in clauses and sometimes shabby marketing tactics of the Big Six.  This is more than just a re-selling exercise of the kind done by Marks &amp; Spencer, which simply plonks its brand name on a service provided by Scottish &amp; Southern. The Co-op is acting as principal, going into the wholesale markets and shouldering serious risk.  It won't be easy, especially since it wants to eschew power from dirty coal-fired stations and favour renewable sources of energy. But if it persuades serious numbers of customers to defect, it may do more to improve industry behaviour than a dozen limp initiatives from Ofgem.