I have a full time job but I've been helping my local pub landlord with some website design services for the last 6 months. So far he's paid me just over £500. Is this taxable and should I be registered with the tax man?
It depends on what your intention is going forward. Do you intend to carry on offering services to the landlord or anyone else? If so then you are holding yourself out for business and therefore self employed.
If not then the income is still income that is untaxed and should be. So you should be preparing a tax return to declare it (regardless if you decide you are self employed).