Rawson:
I sympathise but I'm sorry I have to be harsh to be helpful. The only bill worth discussing is a bill in which the previous readings and the current readings are all actual readings. Is that the case for the £434.28 bill? Any time a bill is based on estimated readings you are entitled to provide actual customer readings and ask for the bill to be recalculated.
You don't say what form of heating you have. Is it gas central heating? Also how efficient is the heating system and how well insulated is the house? Without these answers the numbers are meaningless.
It may still be possible to save money. Make sure you are on the best EON tariff. If not it just takes a phone call to switch to a better EON tariff. And actually it needn't be a hassle to change suppliers. It's a customer right that's worthless if customers won't excercise their right to better value even if the customer service is indistinguishable.
As the previous poster suggested, work out your energy consumption and do comparisons. Once you have a handle on your consumption you are in a better position to challenge the EON "take it or leave it" monthly payment. Turn the tables, if you switch suppliers it costs EON money to replace you as a customer.