Ganapati is OK if you're one of the braying wannabe hipsters who slithers around the environs of Bellenden Road (and are happy to pay £7.50) - otherwise, good dosas can be found in countless places across Tooting and north/west Croydon (Curry Hut in Thornton Heath was where we went when I was a teenager). Ragam in Fitzrovia is good if you're more central. Out in the east, try Dosa Rani in Goodmayes. My new manor of Lewisham has several good Sri Lankan spots which I dare say could rustle up a decent one.