Just flagging this up, thanks to campyoldy.
So far the only Central London bicycle jumble sale is at Look mum no hands! in Clerkenwell:
>> On the first Sunday of every month at Lookmumnohands, Old St. London EC1V 9HX
Sellers: bring as much as you can carry, no van loads
Buyers: 11.00
Contact: Lewin Chalkley 07977 451507 <<
If you have other bike jumble sales you know of, email retro33uk@yahoo.co.uk