Here's an review of an interesting sounding book on the topic. Although books obviously don't get peer reviewed in the same way that proper research does, I think it raises some interesting points: http://www.bankrate.com/brm/news/career/20050307a1.asp
Perhaps my initial argument was indeed based on 'la la land' estimates and there are a lot more factors involved that I had considered, but at least the point about the daftness of Southwark's reporting of the stats was right.