The short answer is yes - You do have to register your self employed business with the tax office. Call 0845 9154515 to register or ask for the number for your local Business Support Team for free advice - the line is open 8-8 Mon-Fri, 8-4 w'ends. Note that there is a £100 fine if you don't register within 3 months of starting business.
Although any income from your business is taxable, if you anticipate that you will earn less than a certain amount (it's about £4,500 in 2007) you can apply for a certificate of exception, which means you won't be liable for national insurance payments on your self employed earnings.
MB