What I would say on that front is we've got good data that shows departments get money in the main estimates, then supplementary estimates (A), (B), and (C). The later in the year departments get their money, their parliamentary authorization, the less chance there is that they will spend it. Money that is in mains has a pretty high rate of being spent. It is the same with supplementary estimates (A), which come early in the year. As for supplementary estimates (B) and (C), those dollars frequently lapse.
I'm oversimplifying here, but if you align the mains and the budget, you can see that budget items get into departments' reference levels earlier. The earlier the departments get their funds voted by Parliament, the earlier they can start spending.
That's the big link to lapses.