Hi Gustavo, thanks for the quick reply.
I’ve duplicated the problem online here: http://crashtest.womenslibrary.org.uk/
Both the offline and online test sites show the same error occurring – I’ve left the online site with just the events calendar and ECPro activated, with the twenty twelve theme (I know the ‘testing for conflicts’ guide asks for twenty thirteen, and the forum asks for twenty fourteen – the tooltip doesn’t display correctly on any of the default WordPress themes, in addition to our own, and the widget itself looks clearest in twenty twelve).
(Susan is my boss, btw – all the technical enquiries come from me, Helen 🙂