@advocatux Thanks. At first I had no success. I restarted the udev service
sudo service udev restart
then checked that the cdc_acm kernel had not been removed
lsmod | grep cdc_acm
started the flash tool again
sudo ./flash_tool.sh
but got the same errors as before.
Then I used a different USB port, and the flash_tool started downloading to the bq M10.
I now have Ubuntu 15.04 OTA13 running. Maybe I'll stick at this level of success for a while, before I risk messing up the M10 yet again.