I made a mistake - the DVD DL media I used is DVD+ (not DVD-). My old DVD player (a Toshiba SD1200) says it will not accept DVD +/- R anything, but it does. It accepts DVD -R single layer to date, no problem. Neither Windows Media or Real Player accepts the DVD+R DL disc (or the Toshiba) on my PC (6 months old), and Windows Explorer cannot find any files at all - i.e. the disc is empty.
It looks like a -R/+R battle to me, and I will have to experiment.....? what say anyone else?