Credit not applied to my bill as promised

Hi there - I recently tried to add TNT sports using the black friday deal after chatting with the help team I was promised a credit on my bill to get down to the deal price. I have the transcript saved in writing. Just recieved my "new" bill and on-going package price with no credit whatsoever. I've been charged more than full price for a month as presume was added during previous bill period and on-going package increased also - am I bring charged for it twice?! Not a happy customer - any advice please?