Going through the Snoopium options one by one, I enabled the OpenFont option. This told me it couldn't find SWScrFont.font (13) or SWRulerFont.font (9). Checking in FWFonts I could see they were there, but tried moving them to FONTS: and this did the trick.
I've no idea why this suddenly became a problem for FinalWriter. Perhaps my ill-fated attempt at installing Ghostscript caused a problem, probably by my own clumsiness.
But very very pleased. Only downside is, I've now no excuse to not work on my ebook!
Thank you to all for your help along the way - good to have a resolved thread!
