An old w/c told me years ago that he only ever declared two thirds of his round, a third of it didn't appear in his books. If he had twelve houses in a street, only eight of them were on his books.
Now unless the tax man was to actually follow him and note down every job, and find out all the prices, he was never going to get caught, and as far as I'm aware he never was.
I itemise all my customers and the price paid, he said there was no need to, he would just write down George street £24 Peel street £30 etc. His accountant accepted this.
There is fiddling going on, and we all know there is, massive corporations operate from offshore accounts and pay very little tax, it's wrong but it happens.
Now if anyone tells me that they have never done a one off job and forgotten to put it down, well they have got a better memory than me.