you may have sorted this by now, but we were with Eon. One day they sent us a letter saying (between gritted teeth no doubt) that they were obliged to point out we could get a better deal elsewhere, so they'd arranged a special rate with Npower.
So I rang Uswitch as suggested, and they said they could actually get us a better deal, and gave me the details over the phone. We ended up with First Utility (now Shell), and saved £500 just like that. I think I had to fill out a simple form but that's all. We didn't have to get a smart meter, just email readings to them every month. Very straightforward.