You may not have worked for 8 months but remember that the 2010/11 fiscal year started on 6 April 2010 (hence you haven't worked for 5 months). This means that effectively when you get paid (at the end of month 6), in addition to 1/12 of your tax code, payroll will also take into account 5 x 1/12 of your tax code (accrued from the 5 non-working months). Any excess is effectively rolled forward if your earnings don't fully utilise this allownce. You could earn £3237 tax free in September.
AJHenderson wrote:Personal Allowance is 6475 for the year or £539 a month
This assumes (likely reasonably) that your tax code is based simply on your personal allowance.