No, you can't plug the black hole in your spending plans with a tax avoidance clampdown

As the May 7 General Election approaches, mainstream political parties are going to place an emphasis on "aggressive tax avoidance," including publicising how much money they plan to raise through stricter policies.  But it's a fiction: They simply have no idea how much money a clampdown on tax avoidance might raise. It's just a handy line in the budget, which can grow or shrink depending on the size of the hole that party needs to fill to "balance the books" on time. And that's important beca...