I changed the permissions to give world readable status to the m3u and everything is now fine. I’m sorry, I can’t believe I didn’t try this before posting.
I’m kind of surprised as the m3u files had the same permissions as the mp3 files (i.e. group readable, but not world readable), and I don’t remember having to do this change before.
However I’m quite happy to change the permissions here (I run a script to “unixify” the paths anyway so to adding a chmod won’t be a big deal).
Thanks, for your speedy reply.