Misleading offering

Dear GHD, my brush broke, it was out of the 6month warranty period. you offer 20% off a replacement and request my old brush. I then send the old brush back and you issue the voucher, when I ask why the voucher is not valid you respond with Ts and Cs that the discount does not apply to brushes. You have misled me from the set go; if I knew this was the case I would not have sent my brush in the first place. Either return my brush or fulfil what you have offered.