Still no joy with this. I've had to resort to telling them to just right click the link and "Save link as..." etc.
Meanwhile the hosts have been looking at it and today said this:
"I've been working on this for most of the day and am unable to find the problem. It seems this is a common problem when using readfile to open a binary file such as PDF or MP3. I have also tried reverting to Apache and the problem still exists, ruling out a problem with Litespeed." (they are using litespeed instead of apache but as you can see even when they reverted to Apache it still didn't work)
Does "common problem when using readfile to open a binary file such as PDF or MP3" mean anything to you?
Your help is really appreciated with this.