In the UK, even if you earn under the tax threshold, you still have to declare self-employed income (there may be a low threshold below which you don't have to declare it at all but I don't know the details so it's a lot safer to check). You won't have to pay tax on it until the amount is above the threshold. However, as you have a full time job you'll already be above the threshold so you'd have to pay tax on any additional income. I think you can earn a certain amount of self-employed income before you have to pay tax on it (not certain so again it's best to check). I agree with Bufty's advice about contacting HMRC, telling them what you've earned and they will tell you if you have to complete a tax return form or not and if you have to pay any tax or not. And they can tell you how to do the form (you can do them online), pay any tax or whatever. If the amount's that small then you probably don't have to pay any tax on it but best to call them and check.
Note: just clicked on your link - as you're in a high tax band already, if there are any thresholds for additional income before you're liable to pay tax, they'll be lower for you than, say, someone on minimum wage who's only just hit the threshold in their main job. So definitely better to check.
Also note: you don't have to declare your self-employed income yet, most self employed people will report it at the end of the tax year, and there is a deadline for when you have to do this. But best to call HMRC now so that you get the info you need to do everything right.
Also, congrats on getting published