Wrongful deduction from deposit

My rental contract stated that a penalty of 2 month's rental have to be paid if you give the Landlord the minimum of 20 working days notice. When I queried this when going in to sign my lease agreement I was told that the amount you pay as penalty will then cover your last month's rental and the other month will be deducted from your deposit. This then implies that if you give two month's notice, the penalty will then cover your last two months of occupation, which Mrs. Knight assured me was the correct interpretation. So, when I decided to end my lease early, I made sure that I gave Daleglen two month's advance notice and I paid the rental for the two months that I was still going to live at the property, but when I received my deposit, they had deducted one month's rental as penalty!! And this after Daleglen had already secured a new tenant only one month after I gave notice. Apart from wrongly interpreting the contract, I can't see how this is a fair practice, since the Landlord had suffered no loss due to the early termination of my Lease. I made Mrs. Knight aware of this by e-mail and tried to phone her twice, but have had no response up to date.