The letting agent is not a law into itself. It acts on the instruction of the landlord. If the lease has not yet ended the letting agent or the landlord can't let themselves into the property. If the lease has ended they can. The strict letter of the law is that all necessary works should be done by the end of the lease. However the landlord can waive or relax anything he wants and can instruct the letting agent to ensure the deposit is returned in full. The letting agent won't benefit or not benefit from the deposit not being returned in full. The landlord might. It sounds to me that the letting agent may have acted incorrectly by going into the flat if the lease was not at an end. However, everything else just displays an ignorance of the informal agreement reached between family members who are respectively landlord and tenant and the agent should have been appraised of this. I hope that helps. Please leave a positive rating so that I am credited for my time.