My guess is that Brown's language has dissuaded them from launching an attack. One of the PM's great skills is to hide bad news behind financial jargon and lingo that don't make sense to the average person. In this case, if the Tories were to major on Brown's "pro- cyclical" admission, then they'd probably be met with a general shrug. Truth is, terms like "pro-cyclical" just don't capture the public imagination.
The solution is to throw the Brown Lexicon out of the window; and not abide by what are, literally, his terms of debate. Instead of saying "pro-cyclical", spell out what it means, so its significance can hit home. As it happens, the London Summit document released by the Government does it perfectly, noting that:
An attack based rather on Brown's admission that his regulatory system "encouraged excessive lending in the good times" could well have some traction...“
"Overall, the regulatory framework had arguably become ‘pro-cyclical’ – encouraging excessive lending in the ‘good times’. The UK Government has asked Lord Turner to consider what reforms need to be made to financial regulation."