Get an accountant
Couldn't agree more.
These days, I'm more familiar with the bastardry of Her Maj's antipodean cousins in the Australian Taxation Office (ATO). Despite what some of these sterling souls might think, there are rules about how much tax one pays. These rules are made by the Parliament, not the ATO. Accountants, and some lawyers specialise in knowing these rules - indeed many of them here in Oz used to work for the ATO and know the score.
As some one who is now self employed, I use one such accountant to handle my affairs. The savings he generates for me are far in excess of his fee - which itself is tax deductible.
I pay what I'm legally obliged to, and no more. Don't take what the ATO or Inland Revenue say at face value. Get professional advice. If that proves to be wrong and you suffer a loss, you can always consider suing the professional concerned.
Incidently, there are processes in Oz to obtain compensation for defective public administration. I'm not sure if that applies in the UK but it's worth checking out.