My experience was almost exactly that of BoingMEL. I had one (main) employer with some occasional work on the side flying scenic trips out of Duxford. Got away with it the first year courtesy of a very good accountant but it was made very clear to me that I'd just scraped by because of the second income. I was told in no uncertain terms that the next year I could expect a thorough investigation. Decided it wasn't worth it, dropped the fun flying and went PAYE. Even then they put me on self assessment where I've been for the last eight years and always get questioned. Nobody bears a grudge like the taxman.
I don't care what anyone says, those dodging tax on the basis that they're self employed (but only freelancing for one operator
) are in for a nasty shock.