A taxing issue for the economy?

I think it’s come to something when even Dave Hartnett, the head honcho of HMRC, doesn’t appear to get basic economics.
Let me say upfront I am NOT advocating non-payment of tax. It is WRONG. But...
If Bob the Builder decides not to tell the taxman when he takes £400 cash in hand for a job, he isn’t “diddling” the economy. What does Dave think Bob will do with the £400? Burn it? Bury it? No – he’ll probably go out and spend it. In the economy.
Now, you can argue that he is depriving the government of money which could be used to fund public spending (though by the time the government’s finished with it I doubt the public will see even half of it). That is where the proper justification lies.
The grey or black economy, however you want to characterise it, is a cheat. It is depriving the needy of help, it probably places a higher burden on people who do pay tax because Treasury calculations about levels of taxation make allowances for non payment.
But it’s wrong to suppose this money simply disappears out of the economy. It probably goes back in as consumer and business spending. So what doesn’t appear in one place probably surfaces in another.
HMRC and the Treasury also have to consider reasons for non-payment. Is it because people are just being dishonest, or is it because a self-employed person doesn’t think it’s fair that he flogs himself for all the hours God gives then has to hand over 40% or 50% or the money?
These are complex arguments. The simple principle of contributing to wider health and wealth through tax is, in my book, unarguable. And not paying any tax is just criminal. But what’s the underlying message for the tax system? Are people being more dishonest? Or are they just feeling the pinch?