Agents are horrible. All of them. Never met a good one. Try every single one in the area and see who seems "sympatico". He also will rip you off, but at least he may be nice-ish. Leave out Leonard Leese or Foxtons.
When I moved to SE1 about 2 years ago we bought our house from an agency called 'Half a percent' (www.halfapercent.com) and i found them to be good and efficient, although I know it's quite different if you are selling, rather than buying. However, the man (James) was really polite and professional and he returned calls, kept promises etc. and generally seemed to know what he was doing and we didn't have any hitches.
I've never used them but I'd avoid Foxtons - dodgy practices.
I bought a BTL flat recently and it was sold via Savills - one agent was nice, the other (woman) a right stuck up northerner was just rude and abbrasive, so I'd avoid their Docklands office as she was just horrid.
Just to note, you don't actually need an estate agent as they don't do anything and as long as you are prepared to do some chasing, a simple solicitor-to-solicitor relationship works fine. You could advertise in Loot or in local shops etc. especially if what you're selling is desirable.
Oh TLMJJ are you really thinking of moving to MCH...woweee how super. Cant wait. What sort of flat are you looking for? I will keep my ears open for things going in the block. Sarahmc has I think just sold hers. I'm not selling mine I fear. But we'll welcome you with open arms.