The commision percentage seems to depend on the location and time of year. (i.e. state of the market). Its always worth getting a few estate agents round.
We are selling DP's flat at the moment, we got four estate agents to quote, and then got the one we prefered to drop to the lowest percentage we were quoted.
All their expenses should be included in the commission and it should be "no sale no fee".
Also be careful, last time I sold a flat they tried to put in that the commision was a percentage of the asking price, not the price that I got for it. (only time I have seen this and struck me as very dodgy). Normally its a percentage of the price it actually sells for.