Thanks agmg, the those references have more or less solved it. The first one was most useful. There are still a couple of wrinkles to sort out, which may be hardware limitations, but the principle is established.
1. It changed the screen size for both the external VGA and the internal DVI screen, so my netbook screen was only showing the top left hand corner when the external TV was "full" screen.
Is this solved when you disable the netbook's screen?
2. "Full" screen on the external monitor wasn't the full width of the TV, there was about an inch of blackness each side of the screen, and the display was horizontally squished slightly, making faces a bit thin! (This is a bit odd as when the TV initially picked up the signal it said it was 1920x1080.)
Same thing happens to my Samsung (only mine is 40'') when set at 1366X768, same as the laptop's monitor. It shows 1920X1080 in the info box but it's not right.
3. The adaptor/TV didn't pick up 1920x1080 if I set that first (no signal), but if I started with the native 1024x600 and then typed in
xrandr --output VGA1 --mode 1920x1080 it switched OK. (I must go back over my commands to see if there was a typo somewhere.)
What's the full set of the commands you use?