Cash Back Not Paid - Stutterfords

I queried a unpaid Cash Back for Stutterfords purchase made in December 2016. Query was sent via web - reference number WRefNo#***#- -MailRef#***# on the 10 March. No reply was received. I followed up with a phone call on Friday 18 March 2016. The consultant proceeded tell me that Stutterfords was no longer a partner, therefore no cash back was paid. I requested confirmation of when this was changed, and proof that communication was sent to me. I cannot located any updated rules brochure relating to this change, which apparently happened in October 2015. I was then left on hold for 30 minutes whilst the agent looked for the communication. She never returned and I eventually cut the call. The call centre agent conduct was unacceptable, the response time to my mail query was also unacceptable (to date no reply). There has been a long history of unpaid cash back from Discoverycard - I am now completely fed up. I demand that this cash back is paid out without delay and Discovery seriously improve track regard with regards to cash back payment.