Hi
Many thanks for the answers, but unfortunately my problem remains unsolved. I've enclosed a screenshot of the settings (there's no export tiddler button in this old beta version):

The setting for 'fullstorywidth' controlled the width of the story when the sidebar was hidden -- seems that this setting was part of the 'vanilla' theme at that time. When these settings are imported into a current version, the other settings each end up in their own settings tiddler, but this one disappears.
This setting does not appear in the theme tweaks tab and I've no idea how this was set. The width of the story river does not, however, expand to fill the full width of the browser window -- and this is the behaviour I'm trying to replicate (if I hide the sidebar in my current version, the story expands to full width.)
How can this be done?
regards