The Letting Centre

Currently we are living in a house, which has now been sold and have received an notice regarding possession. Have been living in the house over 18 months.
When we moved in we paid 1 months rent in advance and a holding desosit of 1 months rent, plus admistration charge and credit checks.
The letting agents have found us a replacement house which we are happy to move to, but the agent now tells us that we must pay £500 as a holding despoit, this is £100 admin charge and credit checks.
My question is why do we have to pay the admin and credit checks again, since we are still with the same agent.  The agent says the deposit is to take the property off the market.