that's per year btw, working from 6 april one year to 5 april the following year.
Tax is cumulative so it's based on the entire year.
NI is different, ie not cumulative, so if you earn lots in one month and pay lots of NI it's tough if you later earn little or nothing.
There is an NI free amount, can't remember how much, but would cover most of the SMP amoutn as i's aound £100 a week, so you pay 11% on anything over the free amount, ie not much as smp isn't much more than £100 per week!