Should people wear a mask in public if they feel unwell with flu?

With NHS and No 10 advice differing, we look at best practice for flu prevention and protection

An NHS leader has said people who are unwell with flu must" wear a mask in public. A UK government official has said if you need to go out while sick, you should only consider" wearing a mask.

So what is the official advice, how serious is the threat posed by flu, and what should you do if have symptoms?