Nimble is robbing us!!!

My account was handed over to Nimble Group and have made arrangements with them to debit my account on the 15th of every month but surprisingly they debit my account on the 13th. I phoned them trying to ask why do they debit my account on the dates we did not agreed on and they said i agreed on the mandate to debit the account on that date for 13th.

I asked them to change the debit order date but i was not assisted but the only thing i saw was a message for a new mandate with a different amount we agreed upon. Why Nimble Group is doing this because i believe this to be *****ulent and would appreciate if this can be corrected or will take it up to ombudsman.

I have also requested for the recordings where they said i agree on a payment date for the 13th but didn't want to send those recordings. Logically who is getting paid on the 13th of each month? Please rectify this.