Members of the public have no idea whether their tax deductions are correct and HMRC call centre officers have virtually no tax training and so are unable to help with any but the most basic queries. Many people are paying far too much tax but unless they pay an accountant to deal with their affairs, they have no chance of getting it back.
I've spent over 35 years working in taxation, firstly for HMRC and now in the private sector and I've seen layer upon layer of legislation added to an already highly complex system. Someone needs to take a fresh look at the whole self assessment regime and complicated rules for everything from pension relief to capital gains, and bring it all back to basics. Perhaps all we need is a high personal allowance and then basic and higher rate tax on the balance with no other allowances or deductions.