Site Sculptor problem resolution
Big thanks to WebAssist and especially to Jason Byrnes for the support ticket and helping to resolve this issue. He was very helpful and a nice chap too, if any of you have the pleasure of talking with him...
RESOLUTION to this Site Sculptor issue:
If anyone else is having a similar problem - here is the summary & resolution of the fault (in my case anyway).
For info, I had recollections of Site Sculptor being a bit erratic in the past too, which may be explained by this as well (although I'd swapped from Windows to Mac along the way, so its hard to tell definitively).
In essence, the extensions may, at times, not run properly if called from within the Preset Manager. Jason's advice was to call these functions separately and then bring it all together in Site Sculptor.
The alternative, which worked, was to open up a separate .php page, use the individual CSS Menu Writer extension in there ('Insert - Web Assist - CSS Menu Writer'), that will write the menu contents & theme in the process, which can then be saved as a Menu Writer preset.
With that done, I could then apply it in 'Preset Manager - Site Sculptor Site Designs', which would then build the site properly, with CMS, when running 'Web Assist - Build Site - New Site Sculptor Site'.
Which all worked fine & dandy and the world was a happier place (mine anyway)...