Sportingbet Casino - Not paying my winnings

I won my bet and proceeded to withdraw my winnings. 10 days passed and I still had no sign of the money in my bank. Nor did sportingbet contact me with an issues. After finally getting in touch with them they then stated that they cant pay my money due to 'a bank charge back'. I have personally never heard of this. They advised me to contact my bank to get a letter to state that the money has left my account to sporting bet. They said any format is acceptable. I got the bank to forward me a letter via e-mail as this is the fastest way. However after forwarding the letter sportingbet failed to contact me for a number of days and then come and said it has to be a physical letter, with every detail about my account on it. I find this very suspicious and my bank did too they were not going to provide this letter. I can see on my bank statements that the money has left me account to sporting bet. But when I ring customer service they just say 'sorry we cant do anything now' This is disgusting and by far the worst customer service experience ive ever had. I won my money fair and square deposited fair and square but now im up the casino will not pay me out.

Please can somebody help me as its close to Christmas and I need my money