Hi Chaps, I totally agree with DPS on this one.
I started my own Limited Company when I went into web design and the extra expense and hassle it caused me was unbelievable. I soon sold on the company (easier than shutting it down completely), paid all accountancy expenses etc off and then registered as self-employed.
Also as DPS mentioned the extra liability placed on the self-employed really isn't an issue here unless you intend to buy/sell hardware as well. Completing tax returns is straightforward enough too as it really is just balancing income against outgoings.
I think you do need to register with HMRC immediately though and there are a few things that your invoice will legally need to show. If your business name does not include your surname you must have your full name on the invoice (i.e. John Smith t/a JS Web Design rather than just JS Web Design).
Here are a couple of helpful links.
Earn an Extra Income When Self-Employed - Extra Income Advice (UK)
Requirements for an invoice | Business Link
Good Luck.