bad service/complaint

I am Mr L. Manxiwa From East London. In March 5 2015 I went for an upgrade on my cellphone contract with MTN. I specifically asked for a top up package on a Samsung galaxy tab which was R349 per month And an extra R140 for insurance. But the consultant failed to set up the usage limit for me and I only noticed when I received a billing statement on the very first month of payment. I went back to the store which the consultant helping told me that the system doesn't allow her to set up the limit for me so she sent an email to the head office and few days after she informed me that the response came saying the problem was resolved. But I kept being billed very highly for 5 months until the MTN call center answered my calls in end July. I since been trying for MTN to credit back my money but no 1 wants to help me. I went as far as speaking to the area manager who admitted the wrong from the store and promised to get back to me but I have not heard from them since. MTN has overcharged me for 5 months and now i am struggling to be credited my money back. The head office told me to deal with the store I really need assistance to this matter as I was wrongly charged. I am in East London