Didn't understand my own tax code and overpaid for 3 years
I'm a mechanic, self employed, been doing it since my late twenties. I do not enjoy paperwork. I do the books late on a Sunday when I'm already annoyed about something else and I just want it done. The tax stuff I kind of treated like a bill that comes in and you pay it and you move on. I knew roughly what I was earning, roughly what I was paying, and those two things seemed roughly connected so I left it alone. I did not know what my tax code actually meant. I thought it was like a postcode for money. Just a reference number. Not something you were supposed to understand or query.
I thought being responsible meant paying on time and not fiddling anything. Turns out there's a third thing, which is checking that you're paying the right amount in the first place. Nobody sends you a letter that says hey by the way you're overpaying, cheers, best wishes, HMRC.