Renewal and ongoing commission

Letting Agents

Hi all,

Currently I have been letting out my flats through agents (high street big chain plus a smaller local one) with rent collection as part of the service, who charge 7% commission taken monthly from the rent, with 5% on subsequent renewals as stipulated in the terms I have signed though with one of the agents the terms are signed again with each new instruction for them to find me a tenant.

My questions are:

1. is this fair contractually/legally for them to forever more be entitled to commission for initially finding me the tenants or is there a cap eg 3 years. They collect rent too (included in their service) but I'm happy to collect directly though there doesn't appear to be an option to do so.
2. if I don't renew and let it go periodic (which many forum members seem to recommend) would I still have to pay commission...ie does a periodic tenancy after the fixed term still attract commission?
3. I can't seem to find London estate agents that only take 1 months rent commission....can anyone recommend agents they use?