Just in case anybody else is thinking of using my proposed .automount unit file mentioned in a previous post, please note that while it does work it also has two unfortunate side-effects:
1) The 'mountpoint' command which was pointed out to me by votdev no longer seems to work - it reports that /sharedfolders/backup is a mountpoint even when the disk is disconnected.
2) If you try to access the shared folder (by using rsync for example) then it hangs for 90 seconds before returning a "no such device" message. This is a lot better than previously when it simply wrote stuff in the wrong place but it would be nice if you could change the timeout interval - I don't believe that you can!
I have yet to find a way to determine if a shared folders is actually available or not other than simply trying and then waiting 90 seconds for an answer!