Thanks to a couple of threads on Folksy that someone else on Etsy pointed me to, I've realised that HMRC are running an amnesty for people who haven't yet registered with them as self-employed but want to get their tax affairs in order.
Find out about it here:
You have to tell HMRC you want to join the amnesty by 14 June 2012, and pay all tax owing by September. I've got a professional exam coming up (one day I'll say enough to exams!) so I can't pre-digest it for you.
Also found through the Folksy/ Etsy threads is HMRC's YouTube channel. Who knew!
Worth a look, there's a video on working out whether you're trading or not...
It's ALWAYS a good idea to get yourself sorted with the taxman sooner rather than later!