Ubasses reply is not strictly correct as there will not be any NI deduction in respect of the pension.
Assuming the person has the basic tax allowance of £7475 per year and only pays basic rate tax at 20%, the gross pension of £1500 will give a monthly income of £1325, allowing for all the tax relief being given against the pension.
To get an income of £2400 per month net, a further £1075 is required. A further monthly income of £1500 gross would suffer £108 NI and £300 Income Tax (if calculated at basic rate - Code BR), leaving a net figure of £1092, which with the pension comes to £2417 per month.
Therefore they would have to earn just under £18,000 per year extra (£1500 per month or £346 per week).