Took what I'd learned in previous posts and applied it to other instances of the id() method call on FrancoToile: specifically the bookmarks, search-in-page and transcripts files. Martin helped, particularly with the two-level call needed in the search-in-page instance.
With Greg also investigated differences in eXist configuration files between the lansdowne and FrancoToile instances - can't find any that explain the different behaviour, so now the mystery is how the lansdowne video site is working.
Have tested fairly thoroughly now against the lansdowne site and appear to be getting identical behaviour.

Still haven't investigated whether any of this will help with the problem encountered on the Map Of London site.
Next will be adding the advanced search GUI for filtering videos including grabbing the legal values for each control, and writing the xquery based on the state of those GUI controls