I've managed to cobble together another website. Since the first site last week, I've installed Netscape 4.75 on my PC, so I can test pages. Style sheets seem to be very wobbly on this release. I'm aware that CSS support in older Netscapes was dodgy, but this problem seems to relate to my pages actually not finding the style sheet file, even though MSIE finds it perfectly.
I guess the answer is out there somewhere. In the meantime, I've also got hold of Netscape 7, and Opera 7.11. If only I could get RISC OS browsers!