After much Googling, reading, hacking, downloading applications and generally fiddling about, we finally got a USB key to put up the correct autorun prompt when you put the key in the port. i.e. put the application you want to run at the top, and not list a load of files and folders to open. It is:
In autorun.inf you want:
And that seemed to do it. Maybe everyone knows how to do this already and just didn’t tell me, but it definitely didn’t work without the ‘action’ line. Okay, the executable still has to be picked from the list of actions, but it appears at the top so it’s fairly obvious.