Oh come on you guys and gals!
TBH I really don't know what the implications are of being both an employee and self-employed at the same time - I've never done that - either one thing or the other.
You'll need to declare your (non-PAYE) earned income on your tax return, and it would be a good idea to talk to HMRC so they can send you one if you don't get it anyway. You're paying NI through your employer, so you may not need to pay a Class 2 contribution as well - ask about that too.
Other than that, I don't think it's terribly complicated as long as the self-employment is not your main source of income. You should be able to pay the additional tax twice-yearly, or get HMRC to adjust your tax code so it's deducted from your salary. I'd go for the first option (if available).
Shock horror, but I've always found the tax people to be very helpful, especially if they think you're trying to play straight.
Another thing to consider is if you should be VAT registered, which depends on whether your customers are mainly (VAT registered) companies or Joe public.