[JonnyM]

No you don't have to register as a business but you may need to inform your employer if working outside of the company isn't allowed as stated in your contract, or moonlighting as it is colloquially known. My employment contract for example categorically states I cannot work on my own account without my employer's permission.

I agree with Mark that any earnings earn on top should be declared; working cash in hand undeclared is a moral and ethical issue (oh, and it can be illegal!). What you earn before paying tax will be on your payslip/P45 etc. That figure is usually reached if in full time employment.

I enquired about this subject recently and had some professional advice from a local company. The 'direct.gov' website is very useful if you haven't tried it yet.

Jonny.