Firstly, did you complete the money transfer within the deadline? It’s typical that such offers are time limited and even time limited for a brand new card. For example my Tesco card had 0% for 24 months but only for money transfers and balance transfers applied in the first 90 days.
If you have the letter you received, it’s a legal document and can be used as a stick to beat them with. Photocopy it and escalate the complaint enclosing the copy of the letter. If they still deny it send your complaint to the financial ombudsman service.
If you don’t have the letter then it’s your word against their’s and unfortunately complaining is doomed to failure. In that case put it down to experience and in future keep letters safely filed away.