Hi Stuart
To add to Simons answer, maybe these graphs are a bit more enlightening: http://overpass-api.de/munin/other-day.html to see the load of the server.
You can also query your own status at https://overpass-api.de/api/status
Generally, you get better success getting a slot on an overcrowded server by making smaller queries. So maybe divide your query into square bboxes and wait some seconds in between until you are granted a new slot.
Hope that helps Michael