Eight million households are to receive advice on how to cut energy bills by an average ?100 as the Government tries to shield people from high fuel costs.  The "big six" energy companies are to send letters spelling out the savings to be had from switching to direct debit payments and dual fuel bills. They will also pledge to spell out on bills whether customers are on the best tariff while making it easier to switch suppliers.  Separately, the Government will write to four million vulnerable households who are eligible for free insulation, urging them to take up the offer.  David Cameron and Chris Huhne will announce the moves at an "energy summit" today, attended by the biggest energy companies, consumer groups, ministers and the regulator Ofgem.  The Prime Minister and the Energy Secretary said last night that the summit marked the start of a more active government role designed to make the energy market more "trusted, simple and transparent" for consumers.  The companies will use today's summit, at the Department of Energy and Climate Change offices, to insist they are serious about transparency and helping customers navigate their way to better deals.  However, Ofgem figures showed last week that the profit margin for energy firms had risen sevenfold over the past four months, to ?125 per customer per year.  4m households could benefit from free home insulation Source: Ofgem