No bluetooth in Settings -> Devices
And bluetooth generally not present anywhere on the machine.
In device manager:
Other devices -> "Bluetooth USB Host Controller" has a warning icon. And there was no info about the device. No manufacturer etc.
Trick was to update the drivers and then point it at the "AppleBluetoothBroadcom64" folder -- which I already had from previous times I faced this issue.
Previously I have downloaded all of the BootCamp drivers, and unzipped them to here:
So I updated the drivers and browsed to that location.
Then it believed that bluetooth was A-OK again.