Letting agent charging excessive renewal fees

Letting Agents

I have a letting agent managing my property on a "let only" basis for an 8% fee.
They found a tenant to occupy the property in October 2020 and have just agreed a lease renewal with them. They have sent me an invoice for 6% of the coming year's rental as a "renewal fee".
The agent has resent me a copy of a Terms of Business I signed with them in April 2017 when I appointed them to manage a different property, outlining the renewal agreement fee clause, which states that the renewal commission fee is due whilst the tenants remain in situ and for 2 years' of renewals after the initial term.
Can you advise whether these types of charges permitted, as the agent has done no work other that to update the lease agreement with the new higher rental rate and arrange for all parties to sign it?
I had also read recently in this...-case-costs-foxtons-4-4-million/
I look forward to your thoughts.
Thank you