As of 25/07/2014 this is currently the UK's cheapest fixed price energy tarrif.
I've just switched my self after finding them on the excellent Money saving expert price comparison "Energy Club" site: https://cheapenergyclub.moneysavingexpert.com/
Feel free to get a quote for your own usage.
Also Co-Op are TOP RATED (by the above site) supplier so that gives some additional confidence.
I've just saved around £80 by switching from my previous supplier based on a number of factors - one of which being I was on my old suppliers "standard tarrif". So if you've not done a price comparison recently get your existing annual usage from you your existing supplier, and then go on the price comparison site to see what savings .
More tarrif info here: http://www.cooperativeenergy.coop/2014/07/17/co-operative-energy-launches-ground-breaking-self-serve-energy-tariff/
Please note this is one of my first postings - so any errors or ommisions - be kind :)