I'm just in the process of switching from first utility to OVO. I got annoyed with first utility because they keep changing my monthly payments even though our usage was a smooth curve with only a few £ per month difference from the previous year and despite me giving them readings each month.
Why increase my payments to £80 (about right if you add up all usage for the year and divide by 12), only to decrease them to £62 (electricity alone is ~£50 a month) a few months later when summer hits. Imagine my complete non shock when they had to increase my payments to £90 this month to cover the shortfall.
What really annoyed me though was their crap responses to my complaint. The last one was a copy paste from their FAQs completely ignoring everything I'd written.