I am filling in a tax return for the first time ever and I have no idea why - I earn nothing expect through my main job which is PAYE. But I've been asked to fill one in, so am doing so. I've entered all the information - which is basically just my salary tax, plus a tiny amount of interest on my savings account - and on the final sheet before I file it, it says 'total amount due' and a sum in the region of £5k. It says this will add this to my statement of account. What does this mean? Does it mean I owe them this money, or they owe it to me? And if so why?