phone ur local tax office and play dum as poop and ask for an appointment, as u dont understand the form and they will go throught it with you,  keep every receipt in a easy to find form,   i use to keep a spread sheet of what i spent on what, ie fuel,  uniforms, etc  go down with this and they will tell you what u need to put in what box,and as the end of it they will tell you what u owe,  if you can get the local tax office on your side, u can make your life a lot easier,  it is better to do this than have some little hitler turn up to ur house and screw you over
i know this because years agoi got chased by the tax man, never filed returns or anything, got fines etc,  went down one day, cried and give a sob story, the women in there filed in my tax return with the info on her screen and i got 600 quid back,  they 0ahve info on you,  if u r working they just want you to confirm what they got to see if it matches
if you are self employed and dont file a tax return, they will just pick someone else and you pay the same tax as them