Small mobile dev in free time - tax on income from apps?

[tintwistle]

Hey! A month or two back I started releasing some apps I had made mostly for personal use but decided they might be worth something to someone. And it turns out they are, I've made a small fortune off Google Play. Now I'm getting concerned about HMRC coming after me for undeclared tax - I understand there is a process to declaring income but really don't have a clue how it works. I'm hoping someone here has an answer for me, and if so I'd be extremely grateful. I've been looking at this tax calculator and combining my normal income with the income from apps - am I doing that right? If so, then I know exactly what is paid by PAYE, and what I need to pay, just not how to do it.

 

If you need any more info to give a better answer please let me know! 

 

Thanks in advance!