Like seemingly every other utility company, Powergen have hiked their gas and electricity prices. There's also an invisible hike in the Powergen bill. They charge units based on tiers: the first so many at one rate, then the next so many at a lower rate. With effect from 10 March 2006, they have raised the threshold at which they change between tariffs from 200 units/month to 225/month. This means you end up paying more units on the first more expensive tier!
Also this morning having taken a close look at the bill received today, they managed to charge me too many units on the first tier (higher rate) even with the increased threshold, in their favour.
Check your Powergen bills (but you might want a PhD in maths and a nose like Sherlock Holmes to root out inaccuracies).