The tax return is very easy to do if your income from self-employment is derived from a 'turnover' (i.e.your gross income before deducting any expenses) of less than £73,000.
However the actual registration process is a bit tedious. You can't even register until AFTER you've started self-employment. You then need to create a 'Government Gateway account' (online) and await postal acceptance, at which point you have to confirm the details of your account by going online and entering the registration code which was posted to you. You then wait again until receiving a letter which provides you with your UTR (Unique Tax Reference).
Thereafter, thankfully, thinks get simpler! Just keep records of all your income and expenses and calculate your gross profit. (HMRC don't require a full statement of your accounts). You can then submit your tax returns online or by post:
http://www.hmrc.gov.uk/selfemployed/
Chris