Recently moved to England - renting property - paid deposit now asked for Tenancy Agreement Fee?

Hi - my sister has recently moved from Scotland to England and paid a deposit and months rent on an apartment but now a few days before moving in she has been asked to pay a Tenancy Agreement Fee - i have googled this (as this is not something done in Scotland) and found that it seems this fee is payable by the Landlord to the Letting Agents, not by the Tenant to the Letting Agents - afterall it is the Landlord who has instructed the LA to find a suitable Tenant. - Can anyone enlighten me is this a fee the Tenant has to pay? If it is her responsibility will this be a regular payment i.e., every 6 months when the lease comes up for renewal - or is it a one off payment. Clearly - anyone outwith the UK who does not know please do not answer. Thanks in advance