This could be construed as a contractual dispute. In this instance the direct debit guranratee does not respond

I have a problem with the cancellation of a contract for recurring swimming lessons with a sports centre, 1Life Management Solutions The contract started on 11th November 2021 for a 30mn swimming lessons once a week. The payment was monthly by direct debit on the 1st of each month (£31) with a catch up first payment taken on 1st December 2021 for £50 to cover the period from 11th November to 30th November 2021 and the month of December 2021. The contract continues unless cancelled for convenience. The relevant cancellation clause says "Subject to payment during the Minimum Payment Period (which is 4 months, so I had passed this), Students who pay fees by direct debit may cancel Membership giving no less than one calendar months’ notice.". I gave them notice on the 7th of July 2022 giving an end of contract on 6th August, but they have charged me for the full month of August, arguing that they require a "calendar month's notice, and refuse to refund me the period from 7th August to 31st August 2022. I cannot understand the contractual or legal basis of their position. They seem to treat this as if I had given notice on the 1st August 2022. What are the remedies? In the 3rd edition of Halsbury's Laws of England, volume 37, the authors wrote: "When the period prescribed is a calendar month running from any arbitrary date the period expires with the day in the succeeding month immediately preceding the day corresponding to the date upon which the period starts: save that, if the period starts at the end of a calendar month which contains more days than the next succeeding month, the period expires at the end of the latter month." So the legal interpretation of a calendar months’ notice is pretty clear. Any views?