Salesmen will always say that their product is the best/cheapest. If you do the comparison on this, or other, sites and can't beat the deal that you've been offered, you should go with what you've signed up to. Otherwise you should investigate whether you can cancel the agreement or not. It is quite possible that you can go with Sainsbury's for a few months before switching again.

Just out of interest, I get nectar points from my provider too. What do I do with them? Are they worth having?

I went to Sainsburys today and was persuaded in store to sign up to Sainsburys Energy Price Protection 2010 tariff for which I apparently get lots of nectar points (up to 4700 per year) and which i was told would be cheaper than British Gas.

I am in a one bedroom flat in the South East - I currently get both Electric and Gas from British Gas standard tariff paying £24 per month direct debit for each. At the moment I have £75 credit on my Electricity and £18 credit on my Gas since signing up at Christmas last year. I am willing to change my provider or try to get a better tariff with BG but am not sure I have made the right choice with Sainsburys, particularly after seeing reviews on this site. The price I am being quoted is (inc VAT):

for gas: Band A - 8.085p, Band B - 3.529p

for electricity: Band A - 21.56p, Band B - 12.13p

What do you think?