I pay tax through PAYE, yet have been asked to complete a tax return? why?

Hi, am hoping someone here can help as when I phoned the Tax office, they were very unhelpful and rude. I am a teacher, have no other income. Have very few savings. About 4k in my current account ( a few pounds of interest over the yr), 6k in a savings account (which I receive about �150 interest on per yr) and about 6k in a joint account with my husband. Nothing else. Yet, when I phoned them asking why I got the letter, they said it was because I had over 10k in savings and was receiving a lot of interest which is cles as an income. Is this correct? What should I do? I am very resentful to pay more tax on my savings since i pay about �700 per month as it is. PLease help and advise me if you can. Much appreciated.