How much for our flat?

With the very unsettled lifestyle we have, Michael and I are planning to move abroad eventually. We have contacted a couple of agencies to get a proper evaluation if we decide to rent it out.
We got in touch with Foxtons, Martyn Gerrard and Kinleigh Folkard & Hayward. I have never had to deal with any of them but they seem to be quite active locally.


We are meeting them in ten days. Any tips on how to pick the right letting agent for us?