Deducting premiums after cancellation

I cancelled my policy on 13 December 2017 and received a letter signed by the COO Wilbur Smith stating: "We refer to your facility and confirm that it has been cancelled with effect from 31/12/2017. Please note that no more premiums will be requested after this date, and that any refund owing will be paid into your account 30 days after the cancellation date." Yet today on 3 January 2018, they deducted another amount (smaller) off of my account. when called they said that this is for the period between the 30th and the 31st of December 2017. When I cancelled the policy, I did ask the person if there will be any additional debit orders and he said no. Very frustrating! I have never been communicated that the insurance policy is for a 30 day period, as they are claiming my prepayment in November was for cover up to 30 December only, this is obviously the case. Very unprofessional service from this company.