This is quite straightforward: you need to register to pay Class 2 National Insurance Contributions (two pounds/week currently). To do this simply call the Helpine for the Newly Self Employed on +44/0 8459 15 45 15. You'll need your National Insurance (NI) number handy. Or check out the Inland Revenue's "Starting Up" site. On that site there is a short postable form if the phone is less appealing.

Watch out for fines for not registering as self-employed soon after becoming self-employed. I didn't realise this and had left it a few months, but didn't get fined thankfully. I did have to pay back to the date when I had started my business, of course.

Some time after registering the Inland Revenue will send you a helpful colorful bound book with I guess about everything you need to know about running a business from a tax perspective.