I have to recommend Marsh & Parsons who have offices in both Balham & Clapham. I've recently used them myself to help with an urgent short-notice Tenancy Agreement issue in Tooting (the previous agent disappeared without a trace!) and I had such a good experience with their manager, Matt that I couldn't not recommend the company now!
Balham & Clapham South office: 020 8673 4377
Clapham office: 020 7501 3666