Why is the amount of tax going to change from month to month when my salary stays the same?

Income tax is payable on the taxable income earned in the tax year, 6th April to 5th April. Income tax is deducted from our pay on a 'pay as you earn' (PAYE) basis. This means that some minor adjustment may be necessary to compensate for slight under or over payments during the tax year.

Of course, taxable allowances might change during the year reflecting a change in personal circumstances, which would obviously result in changes in tax deducted.