Flashing with default - how does the IDE get the .bin file and how is it delivered - could it have been cached? ...was I too quick

Well, all I can think is it's browser caching. When you connect, the web IDE looks at process.env to figure out which board you have, then looks at http://www.espruino.com/json/ESPRUINOBOARD.json. In there it's got the current version and a link to the current firmware.

Having said that, the file is served up with the relevant headers to say 'don't cache' - so if it is being kept it really shouldn't be.