What happens if you're self-emplyed...

[needanothername]

...and you haven't told HMRC about it, but you've actually been doing it for ages. Eg if you started doing something in a very small way, so never thought it necessary, and if you've never gone above the threshold for paying tax or NI so thought it was ok just to carry on. What happens? Do you confess? Would there be a huge fine? Or do you just tell HMRC that you've just started being self-employed?

I would just tell them you've started being self-employed (that's how I did it). I think as long as you don't owe any tax there won't be a problem. If you're a low earner, then it's possible to apply for exemption from paying NI contributions too, so I don't think you've broken any laws.

I think HMRC have bigger fish to fry.