Made a desposit mistake

Tenancy Deposits

Hi,
I have some student tenants who moved in in Sept 2018 and thus i held 6 weeks deposit for each of them into a deposit scheme. These tenants wanted to stay on at the house for an additional year so my letting agent issued them a new contract in Sept 2019. I made a mistake and thought that because they had been living in the house since sept 2018 that i didn't need to refund them 1 week's deposit back. My mistake!
So fast forward to now and i've been chasing one of the tenants for rent arrears and they've got a solicitor to write a letter asking for clarification on the rent arrears and they've also asked how much deposit i am holding as well as asking about whether i have any insurance to cover rent arrears.
What should I do now I realise that i should only have 5 weeks deposit and not 6 weeks? Shall i refund the extra money I am holding immediately and just drop the whole rent arrears thing?
Bit worried as I made a mistake ( my fault). I rang the RLA advice line but to be honest just felt the advisor tried to scare me rather than actually advice what my best course of action should be. Thanks