Thank you, hughbris.
In the end, using the Gantry admin pages without delving into the twig I could only get order: by: title to work ; folder behaved like default with numeric prefixes still attached.
I will think about storing all the pages in question without numeric prefixes and adopting a folder naming scheme to order the pages.
FWIW I echo a previous comment about the documentation being hard to follow in this area (how-to example needed for each option): Content order custom
Ian Russell