I got called in for interview to my tax office when I was self employed.
I was putting invoices through for changing lamps for the elderly and charging very little or nothing.
So when my accounts went through (prepared by my accountant), they smelt a rat.
I was advised by my accountant to still submit invoices to show HMRC that resources were still being used, eg, I may not have charged for the job but I still used Diesel, materials etc....
HMRC were accusing me of not declaring jobs and pocketing cash.
This was my first year of trading and my entire turnover for the year was minute.
But they gunned for me as if I owed them millions.