As mentioned by factor30 there are indeed many factors :D that will affect an individual's Tax and NI liability.
However, if we are to assume "usual" parameters such as Tax Code 522L and no pension contributions or benefits, then to receive �1000 net per month one would need to earn �1245.14 p.m. (or �14,941.68 p.a.) gross, of which:
employee NI = �89.30
employer NI = �103.90
The figures are based on total year earnings therefore the monthly figures can fluctuate by a few pence within PAYE.