bear in mind that any income from part time work is liable to income tax. from memory, you are supposed to tell HMRC you are also self employed (as well), and you will get in to the realms of self assessment and all that sort of thing. you can have a PAYE job and be self employed on part time basis.
This is starting point for tax etc.
income tax should be on net income not gross turnover, so keeping records of expenses (purchasing of stock / equipment / advertising or whatever) is necessary - think you can deduct something for household bills if you work from home, although on the other hand, your mortgage / lease / tenancy agreement / house insurance may also have something to say about this. as might your motor insurers if you use car for business. (and for that matter, think you are supposed to declare to car insurers if you take a second job, in case said job is something they consider 'high risk')
And consider whether whatever you decide to do has a risk of something happening that means you would need to take out public / product liability insurance.
Entirely up to you what you do, but it seems that the tax-person is more eager to chase people making a few quid on the side (e.g. selling a lot on flea-bay) than multinational plcs.
you may also want to check your employment contract to see whether you need permission from your employer to do any other form of work, or whether you need to tell them, or whether you just shouldn't let the part time thing interfere with (or cause a conflict of interests with) your regular job.
have fun