Howard's off-message critique is the most total I have yet seen, particularly on the statistical case against the government's position. It is significant that it came from a former Home Secretary, Tory leader and mentor to David Cameron, who campaigned with a pledge to increase the number of prison places. You can watch it here.
The upshot of these exchanges is that the coalition's rhetoric is self-defeating. Clarke's denial of a link between prison places and the crime rate allows his opponents to strike at him and characterise his view. What the government actually means is that prison doesn't work as well as it should, that causation, prevention and reformation are more important than correlation; or at least that they ought to be. Anne Owers, the former chief inspectatot of prisons, made that point recently using statistics on re-offending, the sense of short-term sentences and prison education schemes that support elements of the government's case.