Thanks for the suggestion. For the record I figured it out. I was calling the Explorer.Search method from within the form I was displaying for the user to construct the query. When the user clicked the button on the form the search executed then the form close. The result was that the search results appeared straight away but the UI did not respond - hence the drop down menu of suggestions as though the search had not been executed.
I changed things so that all the fork did was allow the user to construct the search query. When the user clicked the button the form closed and the Explorer.Search was then executed by code in the main add-in module. Suddenly things worked just as I expected.