UK Landlord Forum / Life

Hi guys new to this forum and hope someone can give me a bit of advice. I've just had to apply for a possession order on my tenants but now they have left. I got a text saying "moved out yesterday keys through door". Later that day I received a letter saying that they had overpaid the rent once in 2012 once 2014 and once in 2015 and I am to refund this money and I have 14 days to respond. Now having checked back with my bank the reason this has happened is they were on housing benefit which I believe is paid 4 weekly and my rental agreement says rent due in advance on 22nd of each month. They did not pay the first rent when they moved in at the start and I did not take a deposit. Before you all laugh these were close friends at the time. When they left the property it is in a bit of a state and needs some work and a lot of cleaning. Can I use any overpaid rent to get this work done.