Sportingbet Casino - Self exclusion complaint

I wish to make a complaint about sporting bet casino who are run and licenced gvc holdings. I have a self exclusion under party casino who are also licenced by gvc holdings. I have been allowed to deposit nearly 900 pounds with sporting bet. Surely this should not have been permitted. I have asked for a refund and explained that they have violated terms and conditions of the responsible gambling act but declined. They failed to understand the seriousness and implications of my complaint as allowing players with a self exclusions with the same licence as another casino can compromise there UK licence as it did for the group cassava who had to refund all players who deposited with a self exclusion in place. I hope you can assist me and take further action for me. I have also sent emails to gvc holding but have had no reply. I have also called them on there customer helpline but no answers as the line doesn't connect. All I've asked for is the money back I should not have been able to deposit. If you need any more information or assistance please don't hesitate to ask