Hi Keith,
I've just come across your thread and I'm away from home for a few weeks.
I think if you view your video waveform at frame rate you'll see why your set is not locking.
The composite sync mode from the graphics card doesn't give totally correct line pulses within the frame pulse. Although rather crude my far from professional piece of design can improve the syncs with a couple of logic devices.
See:
http://www.nostalgiatech.co.uk/Baird%20240%20lines.htm
Peter