I rented a flat with Black Katz West Hampstead from 2008-2009. They were efficient, friendly and professional. We had a lot of problems with the flat when we moved in but they happily helped us get in contact with the Landlord and sorted out the problems we had.
One agent (Jennifer) has been very helpful lately, I lost a copy of my tenancy agreement and she emailed me a copy - very helpful.
I would happily recommend using Black Katz West Hampstead again.