Im 18 months into a 2 year contract, after seeing my latest bill I'm not using hardly any of my tariff, am I right in thinking I can change my tariff? Or is it something I've imagined?
It is possible that you may have an option to move down one step on the tariff ladder (my vodafone contract allows that after 12 months of a 24 month contract) - but it won't save you very much and may trigger a contract extension.
Hold fire until 2 months before the end of your contract & then negotiate hard for a cheaper deal by threatening to change networks.
Go to one of the phoneshops on the high street and discuss it, you can usually get a good deal to end the contract or a buyout from another company that will settle your contract.