I had high hopes for that - not least because I have long suspected that I moved / hid / closed the window with the history in it at some point, and haven't seen it since.
But still no luck (in case anyone else wants to try that though, the xml file is in the Application Data/Sql Developer/System#.#.... directory, not the Sql Devleoper directory, which is where I started looking.
While in there, however I noticed the App.Data / SQL Developer/SqlHistory folder, which has nothing in it created in quite some time - a few months. Could it not be saving the history? Could I have I turned it off unwittingly somehow?