I have been using Ludlow Thompson (in Wimbledon) for many years. So far, I think they are the best agents if you want to let your property - they dominate the letting market in Wimbledon and everytime have found excellent tenants for my flat.
Many often complaint that their after rental customer service is not great and I would say that for landlord, what really matters is to find the right tenants at the right price for your property. They are sales focus and I am comfortable with that - I personally do not need to whole after sale excellence even if I need to chase them on invoicing etc...
They are also candid - they will tell you what your property rental rate is and won't lead you to believe that they can rent it at unbelievable rates like some of the more 'up market' rental agencies... (the F people...)
If you are a landlord and want to rent your property, THEY ARE the go to estate agents.