As an update to this, I've just updated to 1001 (why have I had to do this manually for the last two program updates, when I have it set to update the definitions and the program automatically??), and on reboot, the second blue ball has taken over the 'safely remove hardware' icon for some reason.
To be precise, I updated, rebooted, all ok. I then shut down the PC for a while. On starting up, a pop-up from Avast said that my licence (which expires next February) was invalid, and there was no blue ball. I then rebooted, and the two icons appeared. I have removed it for now by using the second ball to 'safely remove' my usb modem. On plugging it back in, I have the usual arrow thing. Whether the ball will return, only time will tell...