Letting agents are the agents of the landlord. As such, they are supposed to act in the interests of the landlord. They get paid by the landlord for that service. Accordingly, they should not take fees from the tenants - unless the landlord expressly agrees to them doing so. I believe the legal position to be that agents must account to the landlord for any fees that they take from the tenants unless the landlord has agreed that they may keep them- ie the letting agent must give those fees over to the landlord.