Don't go NPower though ... we had loads of problems with them and I think we weren't the only ones. We're with Scottish Power now who I believe are a good mix between fairly competitive but good service.
I hate npower - and have told them so - but they still keep ringing us trying to persuade us to go back . If they can afford to give a £50 dual fuel discount, maybe they should be charging less in the first place?
I liked Scottishpower, we're still with them for gas, but have switched to ecotricity for green electricity.
we use basicpower for electricity, they're good. We have electric underfloor heating etc. and no gas at all so finding a cheap electricity provider was important - before we switched we were with npower who were absolutely dreadful to deal with and cost a third more than basicpower.