Mobile company not giving what they offered

PeteS3

I recently renewed my mobile contract with Talktalk Mobile, the standard package only had 2GB of data but i need 4GB so they offered to increase the data to give me 4GB of data per month, once the account was set up i noticed that it only have 2GB of data, after contacting them about 5 times this was increased to 3.4GB, but not the 4GB that was promised. i have now been on the phone to one of the manager who has looked into this and they have advised that 3.4GB is the highest the system will let the data go too, and while they accept they offered me 4GB the system will not allow this so basically i either have to accept 3.4GB or leave and find a new supplier...are they allowed just to do this, clearly if it was me that wanted to leave, they would charge me to the end of the contract (11Mths) so are they able to just say we cant do what we promised so tough?