Go to this BBC web page...
http://www.bbc.co.uk/reception/digitaltv/widescreen.shtml
Check out Letterbox Mode...
The text says...
"To change from one mode to the other, you can alter the viewing options on your digital receiver. Please see your receiver's manual for details."
Or -on your remote control look for a button with a rudimentary image of a TV screen on it - and press it.
Good luck...
Peccavi, March 2013