The interesting page is
this one about where their electricity comes from.
Only 25-30% of their energy comes from renewable sources, with about 45% coming from Coal and Gas, and most of the rest from Nuclear. That's probably still better than you'd get the big suppliers like Powergen, but it's doesn't quite fit the wind-powered utopia that they allude to on their front page.
My guess is that it probably is more expensive than the standard suppliers, but by how much I don't know. With the recent increases in gas costs, I'm not sure how much longer that will continue.
I've been thinking a bit recently about what the best thing to do would be with respect to my household's carbon emissions. A useful thing to do would be switch from gas heating to electicity, and switch as much as my electricity supply as possible to renewable and nuclear electricity. Unfortunately as last year we had a loft extension along with a new gas boiler, to tear that out and replace with an electric heating system would probably be prohibitively expensive for me at the moment. However, as the price of gas continues to rise, it may make financial sense in the near future.
I don't believe in any of this 'carbon offsetting' crap. When you burn fossil fuels you're basically taking carbon from where it was trapped in the earth's crust and releasing it into the surface ecosystem. If you then manage to get that carbon absorbed into a tree or something, it's still in the surface ecosystem, waiting to be released into the atmosphere again once that tree is burned or biodegrades. The carbon offsetting stuff just doesn't add up.