Bald Brick - bit of a messy expression, the way I put it. I didn't mean to imply that either smplayer or vlc actually did the mounting, as I'm sure you're right and they don't.
And Just19 - answer to your first question is - simply because when I checked the settings in the PCC I was given the option of adding it to fstab. Lazily, I said OK. But I should have known because a couple of years ago I had a helluva time time with external hard drives not mounting - until I removed them from fstab. After that there was never any problem.
And so to your second question - if I comment out these two entries, everything plays at its should and I get no funny messages.

Solved. Thank you. I'd still like to know why I was getting a message about the DVD's having a file system not supported by my system - but that's just out of interest now. Thank you again.