Can't update firmware

When I eventually managed a connection the Firmware update failed in the same manner.

What's that? You mean what you said before about:

• Firmware updater gets the Bangle's attention for a moment, but then the watch goes back to the clock while the App Loader page just shows a constant whirling arrow, which goes on indefinitely until I disconnect. (Same behaviour on Android and Windows (10, Edge) ).
  
  

That is a strange one... And you definitely left the firmware update window in the forground (not minified or moved to a background tab)?

If you really can't get it going then try the instructions on https://www.espruino.com/Firmware+Update­#android-ios-app about using the Android app and DFU mode. I think the issue you had before is just that the watch wasn't in the proper bootloader mode (even if you put it in, it times out after ~30s so you've got to be quick)