Be carefull how much undeclared cash jobs you do.
If you get inspected they go through your personal bank statements and work out how much you are spending. They dont tell you this of course, and then you have to fill out a form detailing your lifestyle expenditure. They then put the two together and if your lifestyle cost is more than you have drawn out of the bank in a year they then confront you with this and ask you where the rest of your money is coming from.
At this point you have painted yourself into a corner and there is no way out other than the costly one.
Tax inspections can happen to any self-employed person at any time, they can go back seven years and it can drag on for months (18 months in my case).
It's a absolute effing nightmare !