In reply to Fozzy:
I've done something similar to this for about 15 years. If your self-employed income is low, filling in the tax return is actually quite simple (the hard part is getting registered to do it online: start that process EARLY!). If your self-employed turn over is less than a few thousand, you don't have to fill in all the numbers: just state your income, profit and costs.
The online tax return does all the maths for you, so you just have to fill in the numbers.
if you expect the following year's self-employed income to be significantly less than this year's, then choose the option to NOT have the tax included in your tax code.
Theres also an exemption available from National Insurance contributions if your self-employed income is less than a few thousand - make sure you get that. I can never remember which NICs it is though!
Hope that helps!