The problem is that there are not enough fully trained inspectors. Self Assessment was introduced simply to get rid of the most experienced staff. Your tax return details are now keyed into a computer by a minimally trained Revenue Assistant. Inspectors carry out investigations when the computer programme picks up some anomaly or inconsistency. I heard a spokesman for HMRC admit on the Today programme that they needed to start training inspectors urgently, as each one,once trained, generated many times their salary in taxes.
There are many more areas of tax than PAYE, which is , or should be , simple. There are different regimes for the self-employed, corporations, partnerships, inheritance tax, CGT, etc. etc. It takes a minimum of three years to cover the basics - we had to learn 300 tax cases off by heart and most of the contents of the huge manuals, plus enough book-keeping to be able to read a set of accounts. This is followed by several years of further training 'on the job'.
Angela Rayner lashes out and calls Sunak “pint sized loser”.