About 30years ago I worked for a taxi firm that use to take the tax man on home visits, as you get talking they admitted that they know that every business, fiddles their returns, but its the ones who are taking the micky who they go after.
An important point to remember is that you are not the only self-employed person doing your trade, there are many many thousands, and most are nearly almost honest with their returns, by looking at what these people earn they can work out an acceptable level that everyone will be earning for the hours they say they work.
Also what are you going to do with the money you have fiddled, you can't put it into a bank because they will want to see records of where it came from, you can't have a nice new motor or holiday or designer clothes because they are too visable and you have to prove where you got the money from.
So all you are left with, is a new matress, stuffed with money, that you can't use.