So, reliefs which cost £250 million are worthwhile, cost-effective and provide significant benefit, while £250 million has to be raised from charities by reducing the use of other reliefs. But that merely prices the relief involved.  If the Treasury is losing £100 from a 40 per cent taxpayer, that equates to a donation of £250. Potentially, then, there could be a fall of £625 million in charitable donations every year. That is a larger sum than the Big Society Capital Fund which the Prime Minister launched on April 4 to provide "organisations tackling major social issues access to new sources of finance to help them thrive and grow".

Ah, you might say, but that does not mean charities will actually lose any cash. An optimist has to assume donors are willing to donate the same gross amount irrespective of any tax relief. Really?

When Alistair Darling increased his proposed additional income tax rate from 45 per cent to 50 per cent, the Treasury thought it would raise £1,140 million. Today, the Treasury believes that reversing this change will cost only £100 million. The original estimate was wrong by 1,040 per cent. As the official assessment explains, "The underlying behavioural response was greater than estimated previously." Nobody would seriously claim the same behaviour, such as emigration, will follow capping of charitable donations. However, the Treasury's own evidence indicates that in the current climate taxpayers are highly responsive to any increase in net liability. It knows perfectly well charitable income will suffer. "The government will explore with philanthropists ways to ensure that this measure will not impact significantly on charities that depend on large donations." (Budget 2012.)

Osborne explained the reduction in tax rate thus: "No Chancellor can justify a tax rate that damages our economy and raises next to nothing — it is as simple as that." If £100 million of revenue from high earners is "next to nothing", how does he rate charities losing up to £625 million a year?

Question 3. How long before the government drops this daft idea? Cameron is already saying that he will "listen" to critics. But will he make cosmetic concessions, or (as we hope) abandon it altogether?

Full disclosure: Standpoint clearly has a dog in this fight. We are owned by a charitable body, the Social Affairs Unit, of which I am the Director, and we depend on the generosity of a small number of public-spirited individuals and organisations.