What might be stopping this Contextualised Filter work in Block (please)?
Found the answer - a logical issue rather than a question of using the controls.

The key was to reverse the order of bringing in content.  Set the main content-type as galleries, then use an entity relationship (set that up first) on Relationship to bring in fields from images and sort on them.

With this procedure, Views can use a Contextual filter for the Galleries. I will wirtie it out more fully on my blog.