Hate to break it to you but technically you are self-employed.

I was self-employed about 8 years back and am now looking to get back into it now my kids are grown. Check out the Starting Out and Running Your Business sections on the site for up-to-date info but basically:

Anyone running a small business in their spare time, regardless of how many other employed jobs and the hours worked and how little earned, needs to register with the Inland Revenue within 3 months or face a fine. It's best to get in with the IR asap and they're actually really helpful - sent me a pack and everything last time around.

You can earn up about £5,000 (the IR will tell you) a tax year before you start having to pay NI contributions and I *think* it's a similar level for starting to pay tax as well.

Get hold of the IR asap - best to start off on the right foot rather than get into trouble with fines and fees.