Unlimited agent fees if tenant remains

Letting Agents

My contract with a letting agency states that I have to continue paying commission for as long as the tenant remains in the property. So that includes the initial period, which is 3 years in my case, any renewal, hold over, extension etc. The contract is to collect rent only (ie. I manage the property). I signed the agent contract nearly 3 years ago and clearly didn't read all the terms before signing it. The tenancy agreement is now about to go periodic and they are still expecting to collect commission... for doing nothing! It seems hugely unfair to be able to charge unlimited fees for a one off service 3 years ago. And I would like to challenge them on this and trying to guage my chances of success..

I'm aware of the Foxtons case where some terms were deemed unfair. Was this type of unlimited nature of fees not one of these? And if so how are agents still allowed to include these terms in their contracts? It's also unfair on perfectly good tenants that they are evicted because of this term.

Has anyone tried to challenge this or gone through the complaints procedure and property ombudsman to challenge this? And how did you get on?

And finally if no fault evictions are banned, are agents seriously going to be allowed to collect commission ad infinitum?!