I've been on illness benefit for few months last year and because of that my employer kept reducing my wages. At some stage I was earning very little. How would that relate to paid tax, I am pretty sure I've been charged the tax from full wages and not reduced rate. This year I applied for p21 and I have underpayment because they added up illness benefit to my gross pay. The thing is there is a huge difference between my gross pay and my net pay that actually landed on my bank account. Usually when you substract paid tax/usc from gross pay you get net pay, right? But not in my case, did my employer calculated the tax incorrectly? Should I ask for a refund?