Letting Agent fees for a periodic tenancy

Letting Agents

I have tenants that will shortly be at the end of a 1 year fixed term contract.

Tenants have asked to move onto a periodic tenancy instead of another fixed term contract. This is because they are looking to purchase own property and may only need a few extra months and cannot fix for another year.

My letting Agent is saying that they will charge fees to change to a periodic tenancy.

Does anyone know if this is correct and reasonable. My understanding from previous tenancies is that at the end of a fixed term contract the tenancy will natrually move to a periodic tenancy and no need to sign a further agreement and therefore no fees.

Thanks

Nick