Sosha

Cheapest on the bay is 3.5k and that's a track bike so depends how much your time's worth I guess or if you're getting a good deal on the replacement.

Edit: more

http://www.gumtree.com/london/91/54746891.html

http://www.gumtree.com/london/58/54301558.html

http://www.gumtree.com/london/08/54067908.html

http://www.gumtree.com/london/79/53267379.html

http://www.gumtree.com/london/16/53791316.html