You'll pay just under 10% in NI on the extra money if in the pension and about 11.4% in NI if not in the pension. If you're in the pension you'll pay 6.4% of the extra money into the TPS. If in the pension, you'll then pay 20% tax on( the extra money earned minus the TPS contribution). If not in the pension, you'll pay 20% tax on the entire extra amount earned. If paying off a student loan, take 9% off the total extra earnings. Divide the annual extra money after deductions by 12 for the extra take-home amount. As a rough calculation, you'd get about £135 extra per month if in the pension and about £143 extra if not in the pension. You take approx £19 off those figures if you have SLC deductions.