I know of a few people who fell foul of the Inland Revenue - had to submit to an in-depth investigation. You don't need to have done anything wrong, it's the luck of the draw, but it can be extremely traumatising.
If you have an accountant, and you don't have insurance against this, it could cost you thousands, even if you are 'squeaky clean'.
The FSB will provide (free) the services of a tax expert to fight your corner for you. The Revenue and Customs know this, and behave accordingly.
I may never have an investigation, and I declare every penny of turnover, but I still sleep a lot easier knowing I've got the big guns on my side.