You've already had your full tax allowance, so that means that you'll pay tax on every penny of your additional earnings. (Assuming that your total earnings don't take you into a higher tax bracket, that will be at 20%. So that's £1200 out of your £6K gone already).
Annoyingly, if you've got two jobs you have to pay two lots of National Insurance as well. However as your second job would (presumably) be as a self-employed worker you'd only have to pay at the lower rate of £2.40 per week plus 8% of your profits above £5715.
http://www.hmrc.gov.u...ntro/selfemployed.htm
Chris