In an acerbic tweet , @jemimakiss says

Tax evasion costs the UK £15bn while benefit fraud costs £1.5bn, says @carolinelucas. But let's target the poor first eh, David Cameron?

Bones of a good article there; how much does the average benefit fraud case cost, and what's the deterrent effect; what's the average tax evasion case cost, and the deterrent effect there.

You could even follow it up with the massive progress made on tax evasion under the last government, tangentially touching on on the scandal of the Inland Revenue selling its own buildings to avoid tax; you could look at the scandal of successive governments - of all hues -  manipulating jobless figures by use of disability etc.