Soldscotts

I make a little money from mobile app development and was wondering - should I be paying tax on this? It's a small amount, adds up to a few hundred a year, but I know when HMRC gets its claws into you it doesn't let go, and would rather not have that happen! I've had a look at this wage calculator to figure out what I'd pay because it'd go between tax bands with my current income and it says I'd pay something like