I have literally just come across the same issues. Moved into a new rented house (2 bedroom) and the landlord had the account for Eon already set up to transfer over to us. But when we looked at the prices per unit, they were REALLY higher, especially for electricity. They were wanting 23.4p per unit whereas somebody like Southern Electric wanted 15.9p or something like that. Gas prices were competitive, but gas prices are low anyway ~8p per unit.
I switched from Eon immediately as my electricity bill would have been £20 for little over two weeks worth of occupancy and the heating had hardly been on either.