I have ferried a plane for a taxman.
When he was given the option of paying through the school and being charged VAT. A bottle and wad of beer tokens was duly produced.
I think actually they have a minimum amount they think they can recover before they are even allowed to try and investigate you.
I cocked up a VAT return to the tune of 400 quid. Admited it and was told to forget it because it would cost more in processing the paper work to sort it out. If i had tried to hide it i don't know what would have happened.
I think the tenner in the car park is quite common, personally i found it quite embarassing until it was the only way i could fill the car.
MJ