Why do people have to be so sillly. Not you sunjoy, the OC. Yes there is something you can do, well actually two things I think.
The trust deposit you originally gave them was security in the event of you losing keys. You haven’t lost any keys. They decided to replace the keys, so under the terms of your original agreement you are entitled to that deposit back, or to apply the deposit to the new keys. It doesn’t matter what they may have resolved between themselves, you haven’t agreed to that and you are one party to that trust arrangement, in a trust situation one party can’t change the terms unilaterally. It is actually quite a serious matter to appropriate trust money, however small the amount might be.
So I suggest you send them a firmly worded letter stating that you paid that money under a trust arrangement and that they have no grounds to vary the terms of that arrangement, therefore you expect the return of the money forthwith. If they don’t, you will contact Fair Trading and make an application to the CTTT.
Another option is for you to contact your landlord and/or the agent.