I'm glad you got it working.
To answer your earlier question on resolution switching, there's a bank of 4 dip switches near the top left of the Naomi, same side where the VGA port is. The first switch on the left determines 15 or 31khz output.
For VGA cables, IIRC pin 9 isn't normally connected to anything. More than likely it's the quality of the cable. I didn't notice before that you're based in the UK, so I'm not sure if this applies to you, but Extron sells a lot of pro grade video equipment and cables, and you can often find them used online for very cheap prices. Otherwise, you can try to figure out what's the most popular UK based pro video manufacturer, and you can probably stumble upon similar deals. Many companies bought massive amounts of analog equipment for many years, and usually dump them after the switch to digital/HD.
If you have any other questions, PM me or start a thread on the Sega section. I feel like we're probably straying a bit far from the topic of this thread now.