Your advice was perfect. As a future dev I would see a facility that could highlight the open parts of a curve - even with you showing me where the gap was it took some finding!
There are scripts already available to find gaps, as I said I knew that trim 'artefact' means that something had a gap so I used the Select Naked Edges script http://kyticka.webzdarma.cz/3d/moi/#SelectNaked