Tuesday 11 August 2009 10.23am
I was wondering the same, though I've seen the 'platform' near westminster bridge, and now just west of Vauxhall Bridge:
I think the real answer may be here on the Port of London Authority web site
- where it says "On or about 18 May 2009, for a period of about a year, a bore hole survey will be conducted between Beckton and Putney as part of the Thames Water Ltd Tideway Tunnel Project." There's a link to a map there of all the spots.
It seems this thing is called a 'jack up barge' like this one: