Withdrawal

I recently joined moanacasino, deposited £40 and ended up winning £3,500, tried withdrawing but received the following email. We would like to inform you that your withdrawal has been cancelled by the payments department and the amount of £2,974.39 has been removed from your balance due to the breach of the following Terms & Conditions: 63.4 A player with 1 deposit only will be eligible to cashout a maximum of 15x the initial deposit amount. All further winnings will be removed from the account upon withdrawal request. Lastly, the amount of £600 is available in your account. Surely this isn’t legal, I’ve tried arguing this with them but they aren’t having it. Who can I take this up with?