Had another meeting with my accountant (sign off last years accounts). She did wonder if they used different people as benchmarks. She has always said I'm either the most honest windowcleaner on her books or the hardest working she's ever had
She thinks they compare people, MrX has been in business as a sole trader for ten years and regularity hits say 50kwhilst Mr Y doesn't earn enough to pay any tax and has been trading just as long and does the same job. She said it does happen but it's difficult for them to prove what you earn.
I have heard of one guy who got caught years ago and another who got investigated although I think he was legit - just bought a lot of vehicles in one year which triggered off a few bells.
They don't appear to have been back to my site since