Just to confuse things even further, we DO have "spanners" here in the US. But rather than being generic for "wrench", the term refers to a specific TYPE of wrench, used to engage slots or pinholes in specialized types of nuts. Common examples would be used for installing/removing the chuck from a metalworking lathe:

http://www.m-w.com/mw/art/spanner.htm

or the locking rings which hold the glass elements into a camera lens:

http://emirco.net/photo/projects/monocle/images/spanner.jpg