The Budget - Table B4
The big uncertainty in the budget is how accurate the predictions in Table B4 turn out to be. The growth from 2000 - 2007 averated 2 3/4. The government is predicting 1-1.5% for this year, 3-3.5% for 2011 and 3 1/4-3 3/4 for 2012.
If those figures are too high then that has a negative feedback on the rest of the budget. They rely on 1% of business investment in 2012 (and 3/4 in 2011), for example, when the average for 2000-7 is 1/4%.
A fraction of a percent has a big effect on the forecast deficit. The difficulty as usual is not having access to the treasury economic model.