Many thanks – that was fast !
And XQuery over XInclude is now working perfectly for newly-added .bike files.
(For files which had aquired the shadowing xmlns since late 2023, sed -i seems, I think, to be able to clean up by editing ‘in place’, without losing the extended attributes on which bike:// row links depend)
(If anyone wants to try the find and sed incantation I am using, they are welcome to DM me for it)