Deposit retained due to none working hours technicality

I hired a storage unit which yes is a nice and clean and secure storage service. I then had to leave for overseas. Over this time I emptied out my unit with the help of people and the last items that were removed I instructed that the person who had the keys to return to the Stor-Age office. I had as well given 15 days notice which was acknowledged. What happened was the keys were returned to a security guard on site of the premises due to the office been closed on the Saturday. To my knowledge the contract was concluded I then saw a debit order go off from Stor-Age luckily the account was empty. I then contacted the Stor-Age company and requested why I was still being debited and they said the lock was still on the unit. I explained that the keys were returned and the employee said that the keys would be delivered to the office when it was open and that I had given notice and the response was simply the following: "Leaving the unit locked and keys with the Facility Assistant is not permitted. In this regard, we can have your lock cut and retain your deposit."

I don't feel that because their offices were closed on a Saturday that it is my fault and by having an employee offer the client a means to deliver the keys by him dropping them off in the office when they opened it would be any different.