@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.

@Lakotaubp Now I do have a battery problem. I let it run down and now it wont charge, whatever charger I use. I think that that is the end of the road for my OPO - shame. However, that is typical for LiPo batteries. They should never be allowed to completely disharge, or so I understand.

@Lakotaubp Thanks. Will do, but it will have to be tomorrow now. I'm not aware of any battery problems. Thanks for your help.

@Lakotaubp I have the same problem with Debian 9 (4.19.97-v7l+) but I'll try your suggestion.

Using power and volume down together for recovery, all I get is this screen, with which I seem to be able to do nothing.
power and volume up together give me a screen that says "Fastboot Mode"

@Lakotaubp Thanks. i'm using elementary OS 0.4.1 Loki
ADB is not seeing my device, unfortunately, whether I have it in fastboot mode or in its boot screen mode. I've tried different ports and usb leads to no avail. WIN 10 seems to see it though ??

@C0n57an71n That's a shame. I like my Nexus 5.

BTW. It has just come back to life, 2 days after being dropped down the toilet! (Not recommended!)

@C0n57an71n My Nexus 5 rarely reconnects to WiFi, but I find it best to cancel the initial request for a WiFi password, then swipe down from the top and select your normal WiFi. Then it connects automatically. But I'm using 15.04.
My BQ M10 reacts the same.

@Lakotaubp Thanks. I'll try that, if I can get passed the initial boot screen.

@dtarrant I bought an M10 Android and have run UBports for a considerable time now, but it was a real pain to install. You have to follow the instructions extremely carefully. I found bits of the install would only work on Win10 (Probably from memory - installing Ubuntu) and bits on Linux, but I got there eventually. It is a very nice tablet, if a bit limited.
(not used with an external display though, as yet)

@karlitozebc Sadly, I have the same problem on a OnePlusOne. With both Linux and Win 10, I can get to the reboot phase, but no further. I just get the opening:

powered by Ubuntu
supported by ubports
screen.

I've tried over and over again, but I think my 'phone must be bricked. A shame because my Nexus 5 had been running for a couple of years on UB before dying.

Thanks @advocatux and @Lakotaubp. Using Ubuntu 19.04 and UB Ports Installer, I've been able to install 16.04 OTA9 on my bq M10 HD, at last.

However, whereas using bq's Ubuntu 15.04 OTA5(?) allowed me to at least browse the internet, but little else, I now have the original problem back. The bq appears to connect to the WiFi but I cannot get a network connection, so no updates, no browser etc.

I've tried 2 different WiFis and have performed a reset. (Settings Reset Erase and Reset all.) This has made no difference as neither WiFi will work. Although it says it has connected to my Bluetooth keyboard, it has not given a code to type and the keyboard isn't working with the bq10. It seems that the wifi and bluetooth issues are connected.

Is there a WiFi/bluetooth I/O problem with the OTA 8 & 9 and the bq M10 HD? Is there anything else I can try?

Edit. Reading wifi bqm10 it seems that this is an old, but unsolved, issue. However, I don't understand why wifi worked well for months then suddenly stopped. Only installing bq's original Ubuntu did I get it back, but that is such a limited OS.

Edit 2: I can connect through a third modem, a mobile MiFi, so I'm up and running. Thanks for all your help.