Depends on your version of Access. With 2003 (and perhaps 2002) you can use
the InStrRev (in string reverse) function to find the position of the last
blank:
Mid([Namefield], InStrRev([namefield], " ") + 1)
This will give incorrect results for names like "Hans van der Steen" or "Ramon
de Garcia", whose last names are van der Steen and de Garcia. Such names will
require a USB (Using Someone's Brain) interface.