No @ on logitech keyboard

I use a Logitech K800 wireless keyboard on a Mac mini with OS X Lion. The @ - character should be alt - 2, but when I press the two together I get a TM logo instead. No other combination of keys seems to work either. Not being able to type "@" at all is enormously frustrating (I am typing this on a different computer, obviously). So far I haven't found a way to amend this is in System preferences. Any suggestions?