If you are within easy range of NW10 in the next half hour or so, and you haven't very much money and would like a bike for a 4 to 7 year old (approx.) for Christmas, I know where three good ones have been left in the street outside a big house - clearly as an offering for anyone who wants to take them. Click Dear Mona at the top left there and email me something and I'll tell you where they are. This message will self-destruct later. Don't ask me to go see if they're still there, it's up to you to do it all.
Update 24/12/2008: The bikes must be gone by now. I'll have a chance to check in a while and confirm. You never know. ... Actually, there were still two there when I looked.