Well... I'm UK-based but unfortunately as only an accountancy student (not yet qualified) - I'm not allowed to offer advice. I do suggest however that you look at the Inland Revenue's website (specifically the Self Assessment bit) and get in touch with them. Alternatively hire the services of a qualified accountant - there should be plenty in the Yellow Pages locally.... there are a number of regulations regarding self-employment, income, declaring income, national insurance, allowances (personal, disability etc) that you are probably totally unaware of, oh and if you are already employed that just makes it a little more complicated!
There is a thread here somewhere , (search for it ),about tax and adsense (UK). Basically it is like any other business income and as such taxable and must be reported to your tax authorities. No big deal,just keep records of all your income and expenses and approach Inland Revenue for further help to clarify your particular situation ,they will help you and advice you whether you need to be registered as self employed etc... Check out the website . [inlandrevenue.gov.uk...]