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.