We're in the process of doing this at the moment (probably for the same reasons you are!) and we're using Talbot Surveyors in North London. Our experience has been good so far.
FFx
I also used Talbot Surveyors for a valuation for a lease extension of a flat - I think the guy's name was Andrew. He did come with his mother but I think he did a good job and was helpful. That was in 2011 for property in W2. You can also find someone in your post code by looking at the RICS web site.
We're in the process of doing this at the moment (probably for the same reasons you are!) and we're using Talbot Surveyors in North London. Our experience has been good so far.
FFx