Do I need to amend the tenancy end date on a Custodial protection if the tenant signs a new fixed term?

No, this is not required in Custodial. Once a deposit has been registered and paid, TDS will continue to hold and protect it until we are notified that the tenancy has ended. If the tenancy is extended, or you sign a new fixed term with the tenant, you do not need to amend the deposit registration.