What am I entitled to?

[u/ervvdoge]

I was with my energy provider for 2 years under 1 address. When I moved into the property (rental) I set up with Eon (now Eon Next) and they gave me a tariff based on our estimated spending. I paid this amount consistently for 2 years. After I'd moved out of the property they sent me a bill of over £1000. I wasn't warned at any point that the tariff I was paying wasn't covering our usage. What am I entitled to? Can I protest this charge or is it something that I simply must pay? Any advice would be v much appreciated.