I had a similar experience some time ago, a lot does depend on the landlord for how easily this situation can be resolved.

A couple of points I would suggest to ponder, firstly give the landlord a reasonable time frame to respond. If there is no response, write again and close your letter with 'your comments are required by return'

Making the assumption that there were no issues when you terminated your agreement, ie any claims by the landlord for damage etc whereby he may wish to charge for said damage, he has no legal right to withhold your excess payment and should return it without delay. However, be prepared for a whole string of excuses if he is that way inclined.

If a couple of letters does not do the trick, then your next step I would suggest would be to write again, opening your letter with 'Notice Of Intended Prosecution' Follow this with, ' As you have so far failed to respond to my letters, I must advise that, should you fail to return the overpayment of £***.** within 14 days, application will be made in the county court for a judgement against you for the amount outstanding plus court fees and costs'

This will get the attention of some landlords, though not all.

You then have the option, if there is still no response, to either put it down to a bad experience and learn from it, or make the application to the court. A fee will be payable at the time you submit your application, but you should be able to request this be added to the claim.

There is still no guarantee that the landlord would refund your money even with a CCJ. You would have the satisfaction of knowing the court had found against him and that his credit score could be adversely affected, however you would also be down by the additional court fee.

My experience was about 3 letters and 2 months to get my money back, I wish you luck.