Tenancy agreement renewal fee

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A year ago I let a flat through an Estate Agent who charged me the full 10% of the annual rent, which I paid, just for introducing a Tenant. They do not manage the property and they did not even prepare the Tenancy agreement as I prefer doing my own . Their services were limited only, I insist, on the introduction of the Tenant.
Now my Tenants have told me they want to renew the Tenancy and I had a call from the Agency to ask me whether I agreed to renew the agreement for another year. They also said that I could increase the rent etc...
but that in any case I would have to pay them a renewal fee: another 10% or possibly 8% of the yearly rental.
I know this is more or less normal practice for Rental Agencies to charge a renewal fee or at least try to, but surely if their work has been limited to the introduction of the Tenant only I cannot see why I should pay for another year fee (about £1500) just for renewal of a Tenancy agreement when they don't even prepare the paper work themselves.
I told them I was relunctant to pay that fee as I did not think it was justified.
Could you tell me where I stand legally with this issue?
Many thanks
I would like to know