Yes, your studentship does rise slightly every year. You should be able to see what it's going to be for the next year a few months before the increase kicks in, as it should be in the student handbook on the ESPRC website (I was funded by the BBSRC and they always did it this way). You don't pay national insurance or tax. To get a rough figure of what your 'real' salary would be, you divide by 3 then multiply by 4, so if you take home £16416, this is roughly equivalent to a salary of £21888.