Since I posted yesterday I've done some more research and it seems the problem was caused by a 3rd party system settings module. This was designed to allow a lot of fancy finger swipes &c on the magic mouse. Removed it. Further, I needed to check a box in the system settings marked with a circular blue icon with a white human figure on it (don't know the name for this in English, in Swedish it's Hjälpmedel which translates to "aides").
Mouse now scrolls OK again.