Overcharged for Cell Contract

Over two years back I opened a contract with Cell C for a Blackberry phone, I however found that for the length of the contract I was overcharged but never had the opportunity to visit a store and when I finally did a few times the stores were too busy. I just did not want to explain my query over a phone.<br> I enquired the overcharge when I finally had the perfect opportunity at one of their stores, that was empty, and the consultant advised me that the Blackberry service was not discounted on my contract and that I was infact charged additional for this service. I thereafter was forced to deal with their call centre, but despite numerous times logging my query and them advising they will come back to me nothing to date has been finalized, or paid back. I even visited further stores to try and sort the query out.<br> The full-term of contract was overcharged (over 20 months) and even on upgrading I advised the consultant that I am doing it on condition my query will be sorted, but still nothing.<br> HOW LONG MUST I WAIT FOR MY REIMBUR*****T, BY RIGHT THIS DUE TO ME SHOULD BE GAINING INTEREST.<br> Unfortunately I do not have record to the reference numbers as they are on my old phone.