Facet API and views contextual filters (facets block on non-search page)
OK, worked this out myself:

Use the contextual filters prefixed "Search:" in the list (not "Indexed node:". There are 2:
"Search: Fulltext search" and "Search: Indexed taxonomy term fields"

Not sure if Search: Fulltext search is ideal as I'm using it for place names and there are a few with more than one word.