1. If you're paying by direct debit, you can switch to quarterly bills instead. Direct debits end up taking more out every month to be safe, but it can take a long time for direct debits to go down even if you're not using as much. That sounds exactly like what's happened here. If you ask, they should be able to reduce the monthly direct debit cost to reflect your usage.
2. If your bills run from an estimated reading, provide them with a reading, or tell them to come round and do a reading themselves.
3. Switch to another provider. Use comparethemarket.com to find out which one can offer you the best price. I'd personally recommend So Energy or SSE. It will take about a month to close your current account.
4. Check that the period you're paying for is when you were actually there. If it's not, then your contract will say it's not your responsibility.
5. Read more advice here https://www.moneysavingexpert.com/utilities/you-switch-gas-electricity/
