Within Links, when it told me I didn't have a compatible course, it gave me a list of the 2 courses that would be compatible (Devil's Island and Devil's Island (LS) in this example)
The thing is, in my course manager, the Devil's Island (LS) listing already appeared exactly as in the compatibility list.
My file name is the same as in Crissy's list, the only other thing I can think of is was there a difference between UK and US versions of the LS add-on course disks?
Oh well, it's no big deal - there was an alternative, which I used.
I'd hate for it to happen on an event that only has 1 course option, though.
Maybe I'd better go look for that fix Freefaller mentioned - see if it's still available somewhere.
Thanks everyone for trying to sort this out.