Here you go, Merton's Mind may be worth contacting, they are listed as Mitcham and there are a few others if you do a search on the Mind website:
http://www.mind.org.uk/help/mind_in_your_area/56
also found this directory, not sure it is quite what you mean though:
http://www.suttonandmerton.nhs.uk/ec...20Nov%2008.pdf
I will keep looking for something more specific though.