Favourite Bike Shops Ever

Where's the best place for second hand bikes in London? I'm trying to find a nice cheap ish hybrid or similar for my sister.
Gumtree is bike thief central and need to test it out with her so ebay is out of the running.
Anyone know any gems?

Walthamstow recycling project

"Bike sales Recycled bikes are available for members of the public to purchase on the first Saturday of each month between 1-3pm, prices start from £45. Children's bikes are also available from £20."