There's a really good one as you go down towards Battersea High Road - it is on a Sunday afternoon between 1pm and 5pm and is held in the grounds (I think) Battersea Technology College?
Also, there was one in Great Suffolk Street on a Sunday starting early in the morning called Big Den's, held in a car park, although I don't know if it is still going - it didn't get off to a very good start is at started in October I believe and it was very cold and blowy.
Its a kind of semi-secret charity shop and lunch club, trying to provide a few affordable things for less well off locals, at the same time as raising a bit of money for activities and trips. Its on Wednesdays, 10-2 I think (not having the exact time to hand), at St Hughs in the basement of Charterhouse's 'Rainbow Building' in Crosby Row SE1, off Long Lane as easyrecall says. So drop in if you want to donate or buy, or find out more about what Charterhouse does. I think they have difficulty collecting but can sometimes do so by arrangement.