There was no really significant policy change – OK there had been some late-cycle relaxation in budgetary discipline in 2006–7. But mainly it was that the policies that had delivered steady surpluses suddenly stopped delivering.
Obviously we know what happened, the property prices had stopped rising at the end of 2006, capital gains were  BIS Review 143/2010  1  down, transactions had slowed dramatically, construction had contracted. But my point is that as an indicator of the sustainability into the future of the public finances, the SGP rules were not adequate.