Hi, I did some mystery shopping a few years ago, so declared myself as 'self-employed'. I didn't think I had to do a tax return for 13-14 as I didn't actually do any mystery shopping, that year so I was hit with a 100 pound fine. No tax was owed. I'm a student who could really do without this, I'm not even a proper 'self-employed' person! I appealed and it was rejected. On my letter, it says there is a 5 percent penalty on an unpaid payment 30 days, 6 months and 12 months after the due date. So if I didn't pay the penalty for 12 months, is it correct that I would owe around 115 pounds? What if I still didn't pay? I used the calculator on the HMRC website, but that only applied interest to the actual tax bill, which would be 0 in my case, not the penalty, so I'm confused.