Tenant change her mind

Tenancy Deposits

A tenant came to see the house and decided to tke the house. She promised to bring the retainer next day, she phoned to say that she will come next week with the money, I agreed and in the mean time another tenant came to view the house this tenant was also interested in the house and wanted to take it. I phoned the previous tenant to confirm if she still wanted the house and she said that she wanted the house and that she will come with the retainer following day and she did, I got £250.00 from her as a retainer,now she has decided not to take the house. I have have lost the other tenant and one months rent. Do I have to give her money back? It will cost me more money to advertise.
I will appreciate your help
Thanks