Data promised on contract

When i was due for an upgrade with Vodacom i wanted to cancel my contract as another service provider offered a better deal. Vodacom then promised me that they will match that deal. After 6 months the 1G of data that i was promised was reduced to 250MD. After that they then allocated me 1G of data for 1 year. But now the problem i have is that my contract has not expired but i am receiving only 250MB of data. The agreement with my contract was 1G of data. I would like vodacom to reimburse me for the data have purchased the last few month as well as send me a copy of the phone call that i had with the agent promising me 1G of data. This is the 2nd time my contract is not as what has been promised to me and i wont upgrade my contract with Vodacom again.